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1

Studies of Elementary Reactions of Chemical Importance in the Atmospheres of Planets  

NASA Technical Reports Server (NTRS)

The methyl self-reaction was studied at T = 298 K and 202 K and at three different pressures, P = 0.5, 1.0, and 2.1 Torr. The experimental measurements were performed in our discharge flow-mass spectrometer (DF-MS) apparatus. The methyl radicals were generated by the reaction of F with methane. Passing a mixture of molecular fluorine, F2, in helium through a microwave cavity generated the atomic fluorine reagent. The atomic F enters the flow tube through a rear port on the flow tube. The methane reagent enters the flow tube through a movable injector located coaxial in the flow tube. The decay of methyl radical signal was monitored at a mass/charge ratio (m/z) of 15 as a function of the injector distance. To minimize secondary chemistry from the reaction CH3 + F to CH2 + HF the initial [CH4](sub 0)/[F](sub 0) was above 37.0 and typically 100. This ensures a 1:1 relationship between initial [F] and [CH3]. A titration of F with excess Cl2 yields the initial [F](sub 0). Our experimental methodology to accurately measure the mass spectrometer scaling factor, i.e., the relationship between initial signal and [CH3](sub 0) has been improved. Now we measure the CH3 signal decay under exponential decay conditions at low initial [F](sub 0), 3x10(exp 11) molecule/cc, in the presence of Cl2. This minimizes the second-order decay contributed by the CH3 self-reaction and a simple extrapolation of the 1n(signal) vs time plot to t = 0 gives the initial signal. This provides the desired relationship between initial signal at 15 amu and [CH3](sub 0). The resulting calibration is then applied to the observed decay of the CH3 signal at high concentrations of CH3 assuming linearity of this scaling factor.

Nesbitt, Fred L.

2001-01-01

2

Chemical Reactions at Surfaces  

SciTech Connect

Chemical reactions at surfaces underlie some of the most important processes of today, including catalysis, energy conversion, microelectronics, human health and the environment. Understanding surface chemical reactions at a fundamental level is at the core of the field of surface science. The Gordon Research Conference on Chemical Reactions at Surfaces is one of the premiere meetings in the field. The program this year will cover a broad range of topics, including heterogeneous catalysis and surface chemistry, surfaces in environmental chemistry and energy conversion, reactions at the liquid-solid and liquid-gas interface, electronic materials growth and surface modification, biological interfaces, and electrons and photons at surfaces. An exciting program is planned, with contributions from outstanding speakers and discussion leaders from the international scientific community. The conference provides a dynamic environment with ample time for discussion and interaction. Attendees are encouraged to present posters; the poster sessions are historically well attended and stimulate additional discussions. The conference provides an excellent opportunity for junior researchers (e.g. graduate students or postdocs) to present their work and interact with established leaders in the field.

Michael Henderson and Nancy Ryan Gray

2010-04-14

3

Chemical reactions with aerosols  

NASA Astrophysics Data System (ADS)

Chemical reactions of aerosol droplets with vapors are discussed. Examples are given in which liquid aerosols of 1-octadecene of narrow size distribution are converted to 1,2-dibromooctadecane with bromine vapor. It was shown that the chemical reaction in the droplet controls the kinetics of this process. The application of chemical reactions with aerosols to the formation of pure, uniform spherical particles of metal oxides is also described. Droplets of metal alkoxides rapidly react with water vapor to yield well-defined powders. The technique was used to prepare titanium dioxide, aluminum oxide, and particles consisting of both metal oxides. This procedure allows generation of powders of predetermined size and composition.

Matijevi?, Egon

4

Chemical Reactions and Pancakes  

NSDL National Science Digital Library

Students will compare ingredients in two different pancake recipes, then taste the difference. We will talk about the chemical reaction that happened when the recipes are mixed and why there are bubbles in the pancakes.

5

Translated chemical reaction networks.  

PubMed

Many biochemical and industrial applications involve complicated networks of simultaneously occurring chemical reactions. Under the assumption of mass action kinetics, the dynamics of these chemical reaction networks are governed by systems of polynomial ordinary differential equations. The steady states of these mass action systems have been analyzed via a variety of techniques, including stoichiometric network analysis, deficiency theory, and algebraic techniques (e.g., Gröbner bases). In this paper, we present a novel method for characterizing the steady states of mass action systems. Our method explicitly links a network's capacity to permit a particular class of steady states, called toric steady states, to topological properties of a generalized network called a translated chemical reaction network. These networks share their reaction vectors with their source network but are permitted to have different complex stoichiometries and different network topologies. We apply the results to examples drawn from the biochemical literature. PMID:24610094

Johnston, Matthew D

2014-05-01

6

Chemical reaction dynamics  

PubMed Central

Understanding the motions of the constituent atoms in reacting molecules lies at the heart of chemistry and is the central focus of chemical reaction dynamics. The most detailed questions one can ask are about the evolution of molecules prepared in a single quantum state to products in individual states, and both calculations and experiments are providing such detailed understanding of increasingly complex systems. A central goal of these studies is uncovering the essential details of chemical change by removing the averaging over the initial conditions that occurs in many cases. Such information provides an exquisite test of theory and helps paint pictures of complicated chemical transformations. The goal of this Special Feature is to provide a snapshot of a portion of the field of chemical reaction dynamics. Much of the work presented here emphasizes a close interplay of experiment and theory in ways that sharpen the conclusions of both and animate future studies. The articles do not completely cover the rich field of chemical reaction dynamics but rather provide a glimpse of some of the emerging insights.

Crim, F. Fleming

2008-01-01

7

Mass Transfer with Chemical Reaction.  

ERIC Educational Resources Information Center

Describes the organization of a graduate course dealing with mass transfer, particularly as it relates to chemical reactions. Discusses the course outline, including mathematics models of mass transfer, enhancement of mass transfer rates by homogeneous chemical reaction, and gas-liquid systems with chemical reaction. (TW)

DeCoursey, W. J.

1987-01-01

8

Critical Evaluation of Chemical Reaction Rates and Collision Cross Sections of Importance in the Earth's Upper Atmosphere and the Atmospheres of Other Planets, Moons, and Comets  

NASA Astrophysics Data System (ADS)

We recommend establishment of a long-term program of critical evaluation by domain experts of the rates and cross sections of atomic and molecular processes that are needed for understanding and modeling the atmospheres in the solar system. We envision products resembling those from the ongoing JPL/NASA Panel for Data Evaluation and the efforts of the international combustion modeling community funded by US DOE and its European counterpart. Both of these endeavors already provide some important inputs for modeling the atmospheres of the Earth, planets, moons, and comets. However, their applications restrict the choice of which processes to evaluate and the temperature and pressure ranges to cover, thus leaving large gaps that need to be filled. Interestingly, an older evaluation program once filled some of these gaps. Funded by the US DoD in the 1960s-1980s, the DNA Reaction Rate Handbook provided a thorough treatment of numerous types of collisions and reactions that are important in the Earth's lower ionosphere, and the program even provided funding for new laboratory measurements. Other examples could be given, with the on-line resources at NIST being among the best, but most provide a narrower scope or less critical evaluation. What is needed is not a just a list of processes and numbers (i.e., a "database"), but rather serious comparison of the available information and specific statements from independent expert laboratory/theory data providers about what should be believed, what uncertainty to assign, and what is most in need of redetermination. The major topic areas would include the following: 1. Chemical reactions of neutral atoms and molecules in their ground electronic states 2. Ion-molecule reactions 3. Chemistry, relaxation, and radiation of electronically excited atoms and molecules 4. Vibrational and rotational relaxation and radiation 5. Photoabsorption, photodissociation, and photoionization 6. Electron-impact excitation, dissociation, ionization, and recombination 7. Energetic heavy particle excitation and charge exchange

Huestis, D. L.

2005-05-01

9

Chemical Reactions: Investigating Exothermic and Endothermic Reactions  

NSDL National Science Digital Library

This activity is an inquiry-based investigation where students discover the indicators of chemical reactions (endothermic and exothermic) by collecting data and using that data to develop a testable question for further experimentation.

10

Chemical Biology\\/ Chemical Genetics\\/ Chemical Genomics: Importance of Chemical Library  

Microsoft Academic Search

A new field of science, chemical biology\\/ chemical genetics\\/ chemical genomics (cb\\/cg\\/cg) has emerged since the late 1990's, especially in the United States. The NIH Roadmap agenda, Molecular Libraries Screening Center Network (MLSCN), became a drive force to push cb\\/cg\\/cg forward. Cb\\/cg\\/cg studies consist of three methodologies, chemical libraries with small molecules, high-throughput screenings, and computational databases. In this review,

Fumihiko Kugawa; Masaru Watanabe; Fuyuhiko Tamanoi

2007-01-01

11

Monotonicity in chemical reaction systems  

Microsoft Academic Search

This article discusses the question of when the dynamical systems arising from chemical reaction networks are monotone, preserving an order induced by some proper cone. The reaction systems studied are defined by the reaction network structure while the kinetics is only constrained very weakly. Necessary and sufficient conditions on cones preserved by these systems are presented. Linear coordinate changes which

Murad Banaji

2009-01-01

12

More on Chemical Reaction Balancing.  

ERIC Educational Resources Information Center

A previous article stated that only the matrix method was powerful enough to balance a particular chemical equation. Shows how this equation can be balanced without using the matrix method. The approach taken involves writing partial mathematical reactions and redox half-reactions, and combining them to yield the final balanced reaction. (JN)

Swinehart, D. F.

1985-01-01

13

Speeding chemical reactions by focusing  

NASA Astrophysics Data System (ADS)

We present numerical results for a chemical reaction of colloidal particles which are transported by a laminar fluid and are focused by periodic obstacles in such a way that the two components are well mixed and consequently the chemical reaction is speeded up. The roles of the various system parameters (diffusion coefficients, reaction rate, and obstacles sizes) are studied. We show that focusing speeds up the reaction from the diffusion limited rate ~t-1/2 to very close to the perfect mixing rate, ~t-1.

Lacasta, A. M.; Ramírez-Piscina, L.; Sancho, J. M.; Lindenberg, K.

2013-04-01

14

Programmability of Chemical Reaction Networks  

NASA Astrophysics Data System (ADS)

Motivated by the intriguing complexity of biochemical circuitry within individual cells we study Stochastic Chemical Reaction Networks (SCRNs), a formal model that considers a set of chemical reactions acting on a finite number of molecules in a well-stirred solution according to standard chemical kinetics equations. SCRNs have been widely used for describing naturally occurring (bio)chemical systems, and with the advent of synthetic biology they become a promising language for the design of artificial biochemical circuits. Our interest here is the computational power of SCRNs and how they relate to more conventional models of computation. We survey known connections and give new connections between SCRNs and Boolean Logic Circuits, Vector Addition Systems, Petri nets, Gate Implementability, Primitive Recursive Functions, Register Machines, Fractran, and Turing Machines. A theme to these investigations is the thin line between decidable and undecidable questions about SCRN behavior.

Cook, Matthew; Soloveichik, David; Winfree, Erik; Bruck, Jehoshua

15

Strain-induced chemical reactions  

SciTech Connect

The uniaxial displacements at leading edges of detonation fronts in solids create large shear-strains. These bend the covalent bonds, and cause piezoelectric effects, thereby closing the HOMO-LUMO energy gaps, and leading to ultra-fast athermal chemical reactions. Specific examples are discussed: ammonium, azide, and nitrate ions; and PETN. It is shown that their properties are consistent with the proposed mechanism. [copyright]American Institute of Physics

Gilman, J.J. (Lawrence Berkeley Laboratory, Berkeley, California 94720 (United States))

1994-07-10

16

Binary counting with chemical reactions.  

PubMed

This paper describes a scheme for implementing a binary counter with chemical reactions. The value of the counter is encoded by logical values of "0" and "1" that correspond to the absence and presence of specific molecular types, respectively. It is incremented when molecules of a trigger type are injected. Synchronization is achieved with reactions that produce a sustained three-phase oscillation. This oscillation plays a role analogous to a clock signal in digital electronics. Quantities are transferred between molecular types in different phases of the oscillation. Unlike all previous schemes for chemical computation, this scheme is dependent only on coarse rate categories for the reactions ("fast" and "slow"). Given such categories, the computation is exact and independent of the specific reaction rates. Although conceptual for the time being, the methodology has potential applications in domains of synthetic biology such as biochemical sensing and drug delivery. We are exploring DNA-based computation via strand displacement as a possible experimental chassis. PMID:21121058

Kharam, Aleksandra; Jiang, Hua; Riedel, Marc; Parhi, Keshab

2011-01-01

17

Visualization of chemical reaction dynamics: Toward understanding complex polyatomic reactions  

PubMed Central

Polyatomic molecules have several electronic states that have similar energies. Consequently, their chemical dynamics often involve nonadiabatic transitions between multiple potential energy surfaces. Elucidating the complex reactions of polyatomic molecules is one of the most important tasks of theoretical and experimental studies of chemical dynamics. This paper describes our recent experimental studies of the multidimensional multisurface dynamics of polyatomic molecules based on two-dimensional ion/electron imaging. It also discusses ultrafast photoelectron spectroscopy of liquids for elucidating nonadiabatic electronic dynamics in aqueous solutions.

SUZUKI, Toshinori

2013-01-01

18

Learning to Predict Chemical Reactions  

PubMed Central

Being able to predict the course of arbitrary chemical reactions is essential to the theory and applications of organic chemistry. Approaches to the reaction prediction problems can be organized around three poles corresponding to: (1) physical laws; (2) rule-based expert systems; and (3) inductive machine learning. Previous approaches at these poles respectively are not high-throughput, are not generalizable or scalable, or lack sufficient data and structure to be implemented. We propose a new approach to reaction prediction utilizing elements from each pole. Using a physically inspired conceptualization, we describe single mechanistic reactions as interactions between coarse approximations of molecular orbitals (MOs) and use topological and physicochemical attributes as descriptors. Using an existing rule-based system (Reaction Explorer), we derive a restricted chemistry dataset consisting of 1630 full multi-step reactions with 2358 distinct starting materials and intermediates, associated with 2989 productive mechanistic steps and 6.14 million unproductive mechanistic steps. And from machine learning, we pose identifying productive mechanistic steps as a statistical ranking, information retrieval, problem: given a set of reactants and a description of conditions, learn a ranking model over potential filled-to-unfilled MO interactions such that the top ranked mechanistic steps yield the major products. The machine learning implementation follows a two-stage approach, in which we first train atom level reactivity filters to prune 94.00% of non-productive reactions with a 0.01% error rate. Then, we train an ensemble of ranking models on pairs of interacting MOs to learn a relative productivity function over mechanistic steps in a given system. Without the use of explicit transformation patterns, the ensemble perfectly ranks the productive mechanism at the top 89.05% of the time, rising to 99.86% of the time when the top four are considered. Furthermore, the system is generalizable, making reasonable predictions over reactants and conditions which the rule-based expert does not handle. A web interface to the machine learning based mechanistic reaction predictor is accessible through our chemoinformatics portal (http://cdb.ics.uci.edu) under the Toolkits section.

Kayala, Matthew A.; Azencott, Chloe-Agathe; Chen, Jonathan H.

2011-01-01

19

Learning to predict chemical reactions.  

PubMed

Being able to predict the course of arbitrary chemical reactions is essential to the theory and applications of organic chemistry. Approaches to the reaction prediction problems can be organized around three poles corresponding to: (1) physical laws; (2) rule-based expert systems; and (3) inductive machine learning. Previous approaches at these poles, respectively, are not high throughput, are not generalizable or scalable, and lack sufficient data and structure to be implemented. We propose a new approach to reaction prediction utilizing elements from each pole. Using a physically inspired conceptualization, we describe single mechanistic reactions as interactions between coarse approximations of molecular orbitals (MOs) and use topological and physicochemical attributes as descriptors. Using an existing rule-based system (Reaction Explorer), we derive a restricted chemistry data set consisting of 1630 full multistep reactions with 2358 distinct starting materials and intermediates, associated with 2989 productive mechanistic steps and 6.14 million unproductive mechanistic steps. And from machine learning, we pose identifying productive mechanistic steps as a statistical ranking, information retrieval problem: given a set of reactants and a description of conditions, learn a ranking model over potential filled-to-unfilled MO interactions such that the top-ranked mechanistic steps yield the major products. The machine learning implementation follows a two-stage approach, in which we first train atom level reactivity filters to prune 94.00% of nonproductive reactions with a 0.01% error rate. Then, we train an ensemble of ranking models on pairs of interacting MOs to learn a relative productivity function over mechanistic steps in a given system. Without the use of explicit transformation patterns, the ensemble perfectly ranks the productive mechanism at the top 89.05% of the time, rising to 99.86% of the time when the top four are considered. Furthermore, the system is generalizable, making reasonable predictions over reactants and conditions which the rule-based expert does not handle. A web interface to the machine learning based mechanistic reaction predictor is accessible through our chemoinformatics portal ( http://cdb.ics.uci.edu) under the Toolkits section. PMID:21819139

Kayala, Matthew A; Azencott, Chloé-Agathe; Chen, Jonathan H; Baldi, Pierre

2011-09-26

20

Minimum Energy Pathways for Chemical Reactions  

NASA Technical Reports Server (NTRS)

Computed potential energy surfaces are often required for computation of such parameters as rate constants as a function of temperature, product branching ratios, and other detailed properties. We have found that computation of the stationary points/reaction pathways using CASSCF/derivative methods, followed by use of the internally contracted CI method to obtain accurate energetics, gives useful results for a number of chemically important systems. The talk will focus on a number of applications to reactions leading to NOx and soot formation in hydrocarbon combustion.

Walch, S. P.; Langhoff, S. R. (Technical Monitor)

1995-01-01

21

Chemical reactions at aqueous interfaces  

NASA Astrophysics Data System (ADS)

Interfaces or phase boundaries are a unique chemical environment relative to individual gas, liquid, or solid phases. Interfacial reaction mechanisms and kinetics are often at variance with homogeneous chemistry due to mass transfer, molecular orientation, and catalytic effects. Aqueous interfaces are a common subject of environmental science and engineering research, and three environmentally relevant aqueous interfaces are investigated in this thesis: 1) fluorochemical sonochemistry (bubble-water), 2) aqueous aerosol ozonation (gas-water droplet), and 3) electrolytic hydrogen production and simultaneous organic oxidation (water-metal/semiconductor). Direct interfacial analysis under environmentally relevant conditions is difficult, since most surface-specific techniques require relatively `extreme' conditions. Thus, the experimental investigations here focus on the development of chemical reactors and analytical techniques for the completion of time/concentration-dependent measurements of reactants and their products. Kinetic modeling, estimations, and/or correlations were used to extract information on interfacially relevant processes. We found that interfacial chemistry was determined to be the rate-limiting step to a subsequent series of relatively fast homogeneous reactions, for example: 1) Pyrolytic cleavage of the ionic headgroup of perfluorooctanesulfonate (PFOS) and perfluorooctanoate (PFOA) adsorbed to cavitating bubble-water interfaces during sonolysis was the rate-determining step in transformation to their inorganic constituents carbon monoxide, carbon dioxide, and fluoride; 2) ozone oxidation of aqueous iodide to hypoiodous acid at the aerosol-gas interface is the rate-determining step in the oxidation of bromide and chloride to dihalogens; 3) Electrolytic oxidation of anodic titanol surface groups is rate-limiting for the overall oxidation of organics by the dichloride radical. We also found chemistry unique to the interface, for example: 1) Adsorption of dilute PFOS(aq) and PFOA(aq) to acoustically cavitating bubble interfaces was greater than equilibrium expectations due to high-velocity bubble radial oscillations; 2) Relative ozone oxidation kinetics of aqueous iodide, sulfite, and thiosulfate were at variance with previously reported bulk aqueous kinetics; 3) Organics that directly chelated with the anode surface were oxidized by direct electron transfer, resulting in immediate carbon dioxide production but slower overall oxidation kinetics. Chemical reactions at aqueous interfaces can be the rate-limiting step of a reaction network and often display novel mechanisms and kinetics as compared to homogeneous chemistry.

Vecitis, Chad David

22

Nuclear physics reactions of astrophysical importance  

NASA Astrophysics Data System (ADS)

Understanding the origin of elements in the universe is one of the main goals of nuclear science and astrophysics today. Achieving this goal involves determining how the elements and their isotopes formed and being able to predict their abundances. At the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory (ORNL), an experimental program has been established to use transfer reactions (such as (p,d) or (d,p)) to study the properties of many nuclei important to understanding the origins of various elements. Three measurements were done to aid in the determination of the origins of different light isotopes. Big Bang Nucleosynthesis calculations, constrained by the Wilkinson Microwave Anisotropy Probe results, produce primordial 7Li abundances almost a factor of four larger than those extrapolated from observations. Since primordial 7Li is believed to be mostly produced by the beta decay of 7Be, one proposed solution to this discrepancy is a resonant enhancement of the 7Be(d, p)2? reaction rate through the 5/2+ 16.7-MeV state in 9B. The 2H(7Be,d) 7Be reaction was used to search for such a resonance; none was observed. An upper limit on the width of the proposed resonance was deduced. 19F is believed to have formed in Asymptotic Giant Branch stars, but current models cannot reproduce the observed abundances of this nucleus. One of the key reactions responsible for the creation of 19F is 15N(?,?). Therefore, it is important to understand reactions that might destroy 15N, such as 15N(n,?). The magnitude of the 15N( n,?) reaction rate depends directly on the neutron spectroscopic factors of low-lying 16N levels. Currently the measured spectroscopic factors differ from those expected from theory by a factor of 2. A study has been done to resolve this discrepancy using the d( 15N,p) reaction. The spectroscopic factors were all found to be close to unity which is in agreement with theoretical predictions. In novae, gamma ray emission is believed to be primarily due to electron-positron annihilation, though the source of these positrons remains a mystery. The positrons are believed to originate from the beta decay of 18F due to its long half-life (t1/2 ˜ 110 min.). To date, gamma rays from this nucleus have not been observed. Therefore, studies have been made on reactions believed to destroy 18F, such as 18F(p,?) which goes through states in 19Ne. A recent study by Adekola et al. showed that a state at 6.289-MeV in 19Ne, just below the proton threshold, could have a significant impact on this reaction rate. However, the spin of this state could not be determined. To determine the spin of this sub-threshold state, a study of the 20Ne(p,d ) reaction was made using a proton beam on a carbon foil implanted with 20Ne. Due target contaminants, a spin assignment could not be made, but a new experimental design was created and is described in this dissertation.

O'Malley, Patrick D.

23

Plasmon-driven sequential chemical reactions in an aqueous environment.  

PubMed

Plasmon-driven sequential chemical reactions were successfully realized in an aqueous environment. In an electrochemical environment, sequential chemical reactions were driven by an applied potential and laser irradiation. Furthermore, the rate of the chemical reaction was controlled via pH, which provides indirect evidence that the hot electrons generated from plasmon decay play an important role in plasmon-driven chemical reactions. In acidic conditions, the hot electrons were captured by the abundant H(+) in the aqueous environment, which prevented the chemical reaction. The developed plasmon-driven chemical reactions in an aqueous environment will significantly expand the applications of plasmon chemistry and may provide a promising avenue for green chemistry using plasmon catalysis in aqueous environments under irradiation by sunlight. PMID:24958029

Zhang, Xin; Wang, Peijie; Zhang, Zhenglong; Fang, Yurui; Sun, Mengtao

2014-01-01

24

Plasmon-driven sequential chemical reactions in an aqueous environment  

PubMed Central

Plasmon-driven sequential chemical reactions were successfully realized in an aqueous environment. In an electrochemical environment, sequential chemical reactions were driven by an applied potential and laser irradiation. Furthermore, the rate of the chemical reaction was controlled via pH, which provides indirect evidence that the hot electrons generated from plasmon decay play an important role in plasmon-driven chemical reactions. In acidic conditions, the hot electrons were captured by the abundant H+ in the aqueous environment, which prevented the chemical reaction. The developed plasmon-driven chemical reactions in an aqueous environment will significantly expand the applications of plasmon chemistry and may provide a promising avenue for green chemistry using plasmon catalysis in aqueous environments under irradiation by sunlight.

Zhang, Xin; Wang, Peijie; Zhang, Zhenglong; Fang, Yurui; Sun, Mengtao

2014-01-01

25

The effect of chemical reaction on liquid round free jet  

Microsoft Academic Search

In the fluid engineering field, chemical reaction frequently occur. Sodium-water chain reaction in the sodium cooled nuclear reactor can make an explosion that causes the accident during the operation of nuclear reactor. Therefore, it is very important to investigate the characteristic of this chemically reacting type of jet to achieve the reliable design of industrial reactor. Although a number of

Seong Dae Hong; Okamoto Koji; Madarame Haruki

2002-01-01

26

The smallest chemical reaction system with bistability  

PubMed Central

Background Bistability underlies basic biological phenomena, such as cell division, differentiation, cancer onset, and apoptosis. So far biologists identified two necessary conditions for bistability: positive feedback and ultrasensitivity. Results Biological systems are based upon elementary mono- and bimolecular chemical reactions. In order to definitely clarify all necessary conditions for bistability we here present the corresponding minimal system. According to our definition, it contains the minimal number of (i) reactants, (ii) reactions, and (iii) terms in the corresponding ordinary differential equations (decreasing importance from i-iii). The minimal bistable system contains two reactants and four irreversible reactions (three bimolecular, one monomolecular). We discuss the roles of the reactions with respect to the necessary conditions for bistability: two reactions comprise the positive feedback loop, a third reaction filters out small stimuli thus enabling a stable 'off' state, and the fourth reaction prevents explosions. We argue that prevention of explosion is a third general necessary condition for bistability, which is so far lacking discussion in the literature. Moreover, in addition to proving that in two-component systems three steady states are necessary for bistability (five for tristability, etc.), we also present a simple general method to design such systems: one just needs one production and three different degradation mechanisms (one production, five degradations for tristability, etc.). This helps modelling multistable systems and it is important for corresponding synthetic biology projects. Conclusion The presented minimal bistable system finally clarifies the often discussed question for the necessary conditions for bistability. The three necessary conditions are: positive feedback, a mechanism to filter out small stimuli and a mechanism to prevent explosions. This is important for modelling bistability with simple systems and for synthetically designing new bistable systems. Our simple model system is also well suited for corresponding teaching purposes.

Wilhelm, Thomas

2009-01-01

27

Analysis of Symmetry in Chemical Reactions.  

National Technical Information Service (NTIS)

The authors investigated the effect of the symmetry of the electronic and nuclear motion in chemical reactions. The analysis focuses on concerted reactions which are defined to be describable by a single transition matrix. The transition matrix for rearra...

T. F. George J. Ross

1971-01-01

28

Dynamic Reaction Figures: An Integrative Vehicle for Understanding Chemical Reactions  

ERIC Educational Resources Information Center

A highly flexible learning tool, referred to as a dynamic reaction figure, is described. Application of these figures can (i) yield the correct chemical equation by simply following a set of menu driven directions; (ii) present the underlying "mechanism" in chemical reactions; and (iii) help to solve quantitative problems in a number of different…

Schultz, Emeric

2008-01-01

29

2005 Chemical Reactions at Surfaces  

SciTech Connect

The Gordon Research Conference (GRC) on 2005 Chemical Reactions at Surfaces was held at Ventura Beach Marriott, Ventura California from February 13, 2005 through February 18, 2005. The Conference was well-attended with 124 participants (attendees list attached). The attendees represented the spectrum of endeavor in this field coming from academia, industry, and government laboratories, both U.S. and foreign scientists, senior researchers, young investigators, and students. In designing the formal speakers program, emphasis was placed on current unpublished research and discussion of the future target areas in this field. There was a conscious effort to stimulate lively discussion about the key issues in the field today. Time for formal presentations was limited in the interest of group discussions. In order that more scientists could communicate their most recent results, poster presentation time was scheduled. Attached is a copy of the formal schedule and speaker program and the poster program. In addition to these formal interactions, 'free time' was scheduled to allow informal discussions. Such discussions are fostering new collaborations and joint efforts in the field.

Cynthia M. Friend

2006-03-14

30

Stochastic flux analysis of chemical reaction networks  

PubMed Central

Background Chemical reaction networks provide an abstraction scheme for a broad range of models in biology and ecology. The two common means for simulating these networks are the deterministic and the stochastic approaches. The traditional deterministic approach, based on differential equations, enjoys a rich set of analysis techniques, including a treatment of reaction fluxes. However, the discrete stochastic simulations, which provide advantages in some cases, lack a quantitative treatment of network fluxes. Results We describe a method for flux analysis of chemical reaction networks, where flux is given by the flow of species between reactions in stochastic simulations of the network. Extending discrete event simulation algorithms, our method constructs several data structures, and thereby reveals a variety of statistics about resource creation and consumption during the simulation. We use these structures to quantify the causal interdependence and relative importance of the reactions at arbitrary time intervals with respect to the network fluxes. This allows us to construct reduced networks that have the same flux-behavior, and compare these networks, also with respect to their time series. We demonstrate our approach on an extended example based on a published ODE model of the same network, that is, Rho GTP-binding proteins, and on other models from biology and ecology. Conclusions We provide a fully stochastic treatment of flux analysis. As in deterministic analysis, our method delivers the network behavior in terms of species transformations. Moreover, our stochastic analysis can be applied, not only at steady state, but at arbitrary time intervals, and used to identify the flow of specific species between specific reactions. Our cases study of Rho GTP-binding proteins reveals the role played by the cyclic reverse fluxes in tuning the behavior of this network.

2013-01-01

31

Classification of Chemical Reactions: Stages of Expertise  

Microsoft Academic Search

In this study we explore the strategies that undergraduate and graduate chemistry students use when engaged in classification tasks involving symbolic and microscopic (particulate) representations of different chemical reactions. We were specifically interested in characterizing the basic features to which students pay attention when classifying chemical reactions at the symbolic and microscopic levels. We identified the categories that students create

Marilyne Stains; Vicente Talanquer

2007-01-01

32

Chemical Kinetics: Rate of Reaction  

NSDL National Science Digital Library

This site offers an interactive tutorial that emphasizes graphical interpretation of chemical kinetics. The stoichiometric coefficients for a chemical equation are determined by comparing the slopes of concentration-time plots for the reactants and products. This tutorial is coupled to others to further guide the student to a better understanding of chemical kinetics.

Blauch, David N.

33

Microfabricated electrochemiluminescence cell for chemical reaction detection  

DOEpatents

A detector cell for a silicon-based or non-silicon-based sleeve type chemical reaction chamber that combines heaters, such as doped polysilicon for heating, and bulk silicon for convection cooling. The detector cell is an electrochemiluminescence cell constructed of layers of silicon with a cover layer of glass, with spaced electrodes located intermediate various layers forming the cell. The cell includes a cavity formed therein and fluid inlets for directing reaction fluid therein. The reaction chamber and detector cell may be utilized in any chemical reaction system for synthesis or processing of organic, inorganic, or biochemical reactions, such as the polymerase chain reaction (PCR) and/or other DNA reactions, such as the ligase chain reaction, which are examples of a synthetic, thermal-cycling-based reaction. The ECL cell may also be used in synthesis instruments, particularly those for DNA amplification and synthesis.

Northrup, M. Allen (Berkeley, CA); Hsueh, Yun-Tai (Davis, CA); Smith, Rosemary L. (Davis, CA)

2003-01-01

34

Laboratory Studies of Heterogeneous Chemical Processes of Atmospheric Importance  

NASA Technical Reports Server (NTRS)

The objective of this study is to conduct measurements of chemical kinetics parameters for heterogeneous reactions of importance in the stratosphere and the troposphere. It involves the elucidation of the mechanism of the interaction of HCl vapor with ice surfaces, which is the first step in the heterogeneous chlorine activation processes, as well as the investigation of the atmospheric oxidation mechanism of soot particles emitted by biomass and fossil fuels. The techniques being employed include turbulent flow-chemical ionization mass spectrometry and optical ellipsometry, among others.

Molina, Mario J.

2004-01-01

35

Microfabricated sleeve devices for chemical reactions  

DOEpatents

A silicon-based sleeve type chemical reaction chamber that combines heaters, such as doped polysilicon for heating, and bulk silicon for convection cooling. The reaction chamber combines a critical ratio of silicon and non-silicon based materials to provide the thermal properties desired. For example, the chamber may combine a critical ratio of silicon and silicon nitride to the volume of material to be heated (e.g., a liquid) in order to provide uniform heating, yet low power requirements. The reaction chamber will also allow the introduction of a secondary tube (e.g., plastic) into the reaction sleeve that contains the reaction mixture thereby alleviating any potential materials incompatibility issues. The reaction chamber may be utilized in any chemical reaction system for synthesis or processing of organic, inorganic, or biochemical reactions, such as the polymerase chain reaction (PCR) and/or other DNA reactions, such as the ligase chain reaction, which are examples of a synthetic, thermal-cycling-based reaction. The reaction chamber may also be used in synthesis instruments, particularly those for DNA amplification and synthesis.

Northrup, M. Allen (Berkeley, CA)

2003-01-01

36

Semiclassical methods in chemical reaction dynamics.  

National Technical Information Service (NTIS)

Semiclassical approximations, simple as well as rigorous, are formulated in order to be able to describe gas phase chemical reactions in large systems. We formulate a simple but accurate semiclassical model for incorporating multidimensional tunneling in ...

S. Keshavamurthy

1994-01-01

37

Modeling of turbulent chemical reaction  

NASA Technical Reports Server (NTRS)

Viewgraphs are presented on modeling turbulent reacting flows, regimes of turbulent combustion, regimes of premixed and regimes of non-premixed turbulent combustion, chemical closure models, flamelet model, conditional moment closure (CMC), NO(x) emissions from turbulent H2 jet flames, probability density function (PDF), departures from chemical equilibrium, mixing models for PDF methods, comparison of predicted and measured H2O mass fractions in turbulent nonpremixed jet flames, experimental evidence of preferential diffusion in turbulent jet flames, and computation of turbulent reacting flows.

Chen, J.-Y.

1995-01-01

38

Entropy Generation in a Chemical Reaction  

ERIC Educational Resources Information Center

Entropy generation in a chemical reaction is analysed without using the general formalism of non-equilibrium thermodynamics at a level adequate for advanced undergraduates. In a first approach to the problem, the phenomenological kinetic equation of an elementary first-order reaction is used to show that entropy production is always positive. A…

Miranda, E. N.

2010-01-01

39

Chemical Principles Revisited: Annotating Reaction Equations.  

ERIC Educational Resources Information Center

Urges chemistry teachers to have students annotate the chemical reactions in aqueous-solutions that they see in their textbooks and witness in the laboratory. Suggests this will help students recognize the reaction type more readily. Examples are given for gas formation, precipitate formation, redox interaction, acid-base interaction, and…

Tykodi, R. J.

1987-01-01

40

Chemical Reactions in Supercritical Carbon Dioxide  

NASA Astrophysics Data System (ADS)

Utilizing supercritical fluids as environmentally benign solvents for chemical synthesis is one of the new approaches in the "greening" of chemistry. Carbon dioxide is the most widely used gas for supercritical fluid studies because of its moderate critical constants, nontoxic nature, and availability in pure form. One unique property of supercritical carbon dioxide (sc-CO2) is its high solubility for fluorinated compounds. Thus sc-CO2 can be used to replace Freons that are conventionally used as solvents for synthesis of perfluoro-polymers. Another property of sc-CO2 is its miscibility with gases such as H2. Heterogeneous reactions involving these gases may become homogeneous reactions in sc-CO2. Reactions in sc-CO2 may offer several advantages including controlling phase behavior and products, increasing speed of reactions, and obtaining specific reaction channels. This paper describes the following nine types of chemical reactions reported in the literature utilizing sc-CO2 as a solvent to illustrate the unique properties of the supercritical fluid reaction systems: (i) hydrogenation and hydroformylation, (ii) synthesis of organometallic compounds, (iii) metal chelation and extraction, (iv) preparation of inorganic nanoparticles, (v) stereo-selectivity of lipase-catalyzed reactions, (vi) asymmetric catalytic hydrogenation, (vii) polymerization, (viii) Diels-Alder reaction, and (ix) free radical reactions.

Wai, Chien M.; Hunt, Fred; Ji, Min; Chen, Xiaoyuan

1998-12-01

41

Computed potential energy surfaces for chemical reactions  

NASA Technical Reports Server (NTRS)

The objective was to obtain accurate potential energy surfaces (PES's) for a number of reactions which are important in the H/N/O combustion process. The interest in this is centered around the design of the SCRAM jet engine for the National Aerospace Plane (NASP), which was envisioned as an air-breathing hydrogen-burning vehicle capable of reaching velocities as large as Mach 25. Preliminary studies indicated that the supersonic flow in the combustor region of the scram jet engine required accurate reaction rate data for reactions in the H/N/O system, some of which was not readily available from experiment. The most important class of combustion reactions from the standpoint of the NASP project are radical recombinaton reactions, since these reactions result in most of the heat release in the combustion process. Theoretical characterizations of the potential energy surfaces for these reactions are presented and discussed.

Walch, Stephen P.

1990-01-01

42

Reaction of Phenyl Radical with O2: Thermodynamic Properties, Important Reaction Paths and Kinetics  

SciTech Connect

The Phenyl + O{sub 2} association results in a chemically activated phenyl-peroxy radical which can dissociate to phenoxy radical + O, undergo intramolecular addition of the peroxy radical to several unsaturated carbon sites or react back to phenyl + O{sub 2}. The intramolecular addition channels further react through several paths to ring opening (unsaturated + carbonyl moieties) as well as cyclopentadieny radical + CO{sub 2}. Enthalpy ({Delta}H{sub f(298)}{sup o}), Entropy (S{sub 298}), and heat capacities Cp(T) for species in the decomposition of the ring are evaluated using density functional and ab initio calculations and by comparisons to vinyl + O{sub 2} data of Mebel et al, and phenyl + O{sub 2} data of Hadad et al. Isodesmic reaction analysis is used to estimate enthalpy values of the intermediates and well depths of the adducts. High Pressure limit kinetic parameters are obtained from the calculation results using canonical Transition State Theory. Quantum RRK analysis is utilized to obtain k(E) and modified strong collision or master equation analysis is used for evaluation of pressure fall-off in this complex bimolecular, chemical activation, reaction system. Uncertainty in key barriers is discussed, resulting variations in important reaction product ratios are illustrated, and changes in these branching ratios are evaluated with a detailed reaction mechanism.

Bozzelli, J; Sebbar, N; Pitz, W; Bockhorn, H

2001-04-12

43

Memory Switches in Chemical Reaction Space  

PubMed Central

Just as complex electronic circuits are built from simple Boolean gates, diverse biological functions, including signal transduction, differentiation, and stress response, frequently use biochemical switches as a functional module. A relatively small number of such switches have been described in the literature, and these exhibit considerable diversity in chemical topology. We asked if biochemical switches are indeed rare and if there are common chemical motifs and family relationships among such switches. We performed a systematic exploration of chemical reaction space by generating all possible stoichiometrically valid chemical configurations up to 3 molecules and 6 reactions and up to 4 molecules and 3 reactions. We used Monte Carlo sampling of parameter space for each such configuration to generate specific models and checked each model for switching properties. We found nearly 4,500 reaction topologies, or about 10% of our tested configurations, that demonstrate switching behavior. Commonly accepted topological features such as feedback were poor predictors of bistability, and we identified new reaction motifs that were likely to be found in switches. Furthermore, the discovered switches were related in that most of the larger configurations were derived from smaller ones by addition of one or more reactions. To explore even larger configurations, we developed two tools: the “bistabilizer,” which converts almost-bistable systems into bistable ones, and frequent motif mining, which helps rank untested configurations. Both of these tools increased the coverage of our library of bistable systems. Thus, our systematic exploration of chemical reaction space has produced a valuable resource for investigating the key signaling motif of bistability.

Ramakrishnan, Naren; Bhalla, Upinder S.

2008-01-01

44

Laboratory Studies of Heterogeneous Chemical Processes of Atmospheric Importance  

NASA Technical Reports Server (NTRS)

The objective of this study is to conduct measurements of chemical kinetics parameters for heterogeneous reactions of importance in the stratosphere and the troposphere. It involves the elucidation of the mechanism of the interaction of HC1 vapor with ice surfaces, which is the first step in the heterogeneous chlorine activation processes, as well as the investigation of the atmospheric oxidation mechanism of soot particles emitted by biomass and fossil fuels. The techniques being employed include turbulent flow- chemical ionization mass spectrometry and optical ellipsometry, among others. The next section summarizes our research activities during the first year of the project, and the section that follows consists of the statement of work for the second year.

Molina, Mario J.

2003-01-01

45

Runaway chemical reaction exposes community to highly toxic chemicals.  

PubMed

The U.S. Chemical Safety and Hazard Investigation Board (CSB) conducted a comprehensive investigation of a runaway chemical reaction at MFG Chemical (MFG) in Dalton, Georgia on April 12, 2004 that resulted in the uncontrolled release of a large quantity of highly toxic and flammable allyl alcohol and allyl chloride into the community. Five people were hospitalized and 154 people required decontamination and treatment for exposure to the chemicals. This included police officers attempting to evacuate the community and ambulance personnel who responded to 911 calls from residents exposed to the chemicals. This paper presents the findings of the CSB report (U.S. Chemical Safety and Hazard Investigation Board (CSB), Investigation Report: Toxic Chemical Vapor Cloud Release, Report No. 2004-09-I-GA, Washington DC, April 2006) including a discussion on tolling practices; scale-up of batch reaction processes; Process Safety Management (PSM) and Risk Management Plan (RMP) implementation; emergency planning by the company, county and the city; and emergency response and mitigation actions taken during the incident. The reactive chemical testing and atmospheric dispersion modeling conducted by CSB after the incident and recommendations adopted by the Board are also discussed. PMID:18313843

Kaszniak, Mark; Vorderbrueggen, John

2008-11-15

46

Perspective: Bimolecular chemical reaction dynamics in liquids.  

PubMed

Bimolecular reactions in the gas phase exhibit rich and varied dynamical behaviour, but whether a profound knowledge of the mechanisms of isolated reactive collisions can usefully inform our understanding of reactions in liquid solutions remains an open question. The fluctuating environment in a liquid may significantly alter the motions of the reacting particles and the flow of energy into the reaction products after a transition state has been crossed. Recent experimental and computational studies of exothermic reactions of CN radicals with organic molecules indicate that many features of the gas-phase dynamics are retained in solution. However, observed differences may also provide information on the ways in which a solvent modifies fundamental chemical mechanisms. This perspective examines progress in the use of time-resolved infra-red spectroscopy to study reaction dynamics in liquids, discusses how existing theories can guide the interpretation of experimental data, and suggests future challenges for this field of research. PMID:24606343

Orr-Ewing, Andrew J

2014-03-01

47

A DFT analysis of thermal decomposition reactions important to natural products.  

PubMed

The thermal decomposition reactions of several important natural flavor and fragrance chemicals have been investigated using density functional theory (DFT, B3LYP/6-31G*). Retro-aldol reactions of glucose, fructose, hernandulcin, epihernandulcin, [3]-gingerol, and [4]-isogingerol; retro-carbonyl-ene reactions of isopulegol, lavandulol, isolyratol, and indicumenone; and pyrolytic syn elimination reactions of linalyl acetate, alpha-terpinyl acetate, and bornyl acetate, have been carried out. The calculations indicate activation enthalpies of around 30 kcal/mol for the retro-aldol reactions and for retro-carbonyl-ene reactions, comparable to pericyclic reactions such as the Cope rearrangement and electrocyclic reactions, and therefore important reactions at elevated temperatures (e.g., boiling aqueous solutions, gas-chromatograph injection ports). Activation enthalpies for pyrolytic eliminations are around 40 kcal/mol and are unlikely to occur during extraction or GC analysis. PMID:20734926

Setzer, William N

2010-07-01

48

Computer Animation of a Chemical Reaction.  

ERIC Educational Resources Information Center

Taking a prototype chemical reaction (molecular hydrogen plus hydrogen atom), constructs an accurate semiempirical, generalized diatomics-in-molecules potential energy surface, calculates motions of these atoms on this surface using REACTS trajectory program, and presents results as moving picture on a microcomputer graphics system. Provides…

Eaker, Charles W.; Jacobs, Edwin L.

1982-01-01

49

CHEMICAL REACTIONS INDUCED BY UNDERGROUND NUCLEAR EXPLOSIONS  

Microsoft Academic Search

As part of the general program for study of potential industrial and ; civil applications of nuclear explosions (Plowshare Program), chemical reactions ; useful for the recovery of nuclear energy are discussed. Three problems are ; considered. First, the mechanism for the transfer of explosive energy to the ; medium is discussed and the results of calculations of useful explosive

G. H. Higgins; D. E. Rawson; W. Z. Wade

1961-01-01

50

CO Chain-Reaction Chemical Laser Research.  

National Technical Information Service (NTIS)

A chain-reaction CO chemical laser fueled by carbon monosulfide (CS) and molecular oxygen (O2) has been demonstrated. Initiation by the chain carriers (either O-atoms or S-atoms) is necessary; the chain length lambda* increases rapidly with CS/CS2 fuel mo...

W. Q. Jeffers H. Y. Ageno C. E. Wiswall

1976-01-01

51

Chemical reaction engineering using molecularly imprinted polymeric catalysts  

Microsoft Academic Search

Enzymes play an important role as highly specific catalysts in biotechnology [J. Biotechnol. 66 (1998) 3; Hydrolases in Organic Synthesis, Wiley, New York, 1999] as well as in chemical reaction engineering [J. Biotechnol. 59 (1997) 11; Trends Biotechnol. 13 (7) (1995) 253]. However, the drawbacks of these biomaterials are poor durability and relatively high costs of production. Thus, the technique

Oliver Brüggemann

2001-01-01

52

Neutral theory of chemical reaction networks  

NASA Astrophysics Data System (ADS)

To what extent do the characteristic features of a chemical reaction network reflect its purpose and function? In general, one argues that correlations between specific features and specific functions are key to understanding a complex structure. However, specific features may sometimes be neutral and uncorrelated with any system-specific purpose, function or causal chain. Such neutral features are caused by chance and randomness. Here we compare two classes of chemical networks: one that has been subjected to biological evolution (the chemical reaction network of metabolism in living cells) and one that has not (the atmospheric planetary chemical reaction networks). Their degree distributions are shown to share the very same neutral system-independent features. The shape of the broad distributions is to a large extent controlled by a single parameter, the network size. From this perspective, there is little difference between atmospheric and metabolic networks; they are just different sizes of the same random assembling network. In other words, the shape of the degree distribution is a neutral characteristic feature and has no functional or evolutionary implications in itself; it is not a matter of life and death.

Lee, Sang Hoon; Bernhardsson, Sebastian; Holme, Petter; Kim, Beom Jun; Minnhagen, Petter

2012-03-01

53

Computed potential energy surfaces for chemical reactions  

NASA Technical Reports Server (NTRS)

A new global potential energy surface (PES) is being generated for O(P-3) + H2 yields OH + H. This surface is being fit using the rotated Morse oscillator method, which was used to fit the previous POL-CI surface. The new surface is expected to be more accurate and also includes a much more complete sampling of bent geometries. A new study has been undertaken of the reaction N + O2 yields NO + O. The new studies have focused on the region of the surface near a possible minimum corresponding to the peroxy form of NOO. A large portion of the PES for this second reaction has been mapped out. Since state to state cross sections for the reaction are important in the chemistry of high temperature air, these studies will probably be extended to permit generation of a new global potential for reaction.

Walch, Stephen P.; Levin, Eugene

1993-01-01

54

Researches on Preliminary Chemical Reactions in Spark-Ignition Engines  

NASA Technical Reports Server (NTRS)

Chemical reactions can demonstrably occur in a fuel-air mixture compressed in the working cylinder of an Otto-cycle (spark ignition) internal-combustion engine even before the charge is ignited by the flame proceeding from the sparking plug. These are the so-called "prelinminary reactions" ("pre-flame" combustion or oxidation), and an exact knowledge of their characteristic development is of great importance for a correct appreciation of the phenomena of engine-knock (detonation), and consequently for its avoidance. Such reactions can be studied either in a working engine cylinder or in a combustion bomb. The first method necessitates a complicated experimental technique, while the second has the disadvantage of enabling only a single reaction to be studied at one time. Consequently, a new series of experiments was inaugurated, conducted in a motored (externally-driven) experimental engine of mixture-compression type, without ignition, the resulting preliminary reactions being detectable and measurable thermometrically.

Muehlner, E.

1943-01-01

55

Multiscale stochastic simulations of chemical reactions with regulated scale separation  

SciTech Connect

We present a coupling of multiscale frameworks with accelerated stochastic simulation algorithms for systems of chemical reactions with disparate propensities. The algorithms regulate the propensities of the fast and slow reactions of the system, using alternating micro and macro sub-steps simulated with accelerated algorithms such as ? and R-leaping. The proposed algorithms are shown to provide significant speedups in simulations of stiff systems of chemical reactions with a trade-off in accuracy as controlled by a regulating parameter. More importantly, the error of the methods exhibits a cutoff phenomenon that allows for optimal parameter choices. Numerical experiments demonstrate that hybrid algorithms involving accelerated stochastic simulations can be, in certain cases, more accurate while faster, than their corresponding stochastic simulation algorithm counterparts.

Koumoutsakos, Petros, E-mail: petros@ethz.ch [Chair of Computational Science, Clausiusstrasse 33, ETH Zurich, CH-8092 (Switzerland)] [Chair of Computational Science, Clausiusstrasse 33, ETH Zurich, CH-8092 (Switzerland); Feigelman, Justin [Chair of Computational Science, Clausiusstrasse 33, ETH Zurich, CH-8092 (Switzerland)] [Chair of Computational Science, Clausiusstrasse 33, ETH Zurich, CH-8092 (Switzerland)

2013-07-01

56

Some chemical and mineralogical considerations important for understanding leachate chemistry  

SciTech Connect

Both the raw shale material and the retorting process parameters are influential in determining the subsequent behavior of the spent shale solids with respect to leaching and/or environmental weathering. The process parameters define the mineral reactions that occur to form the mineral assemblage in the spent shale, while the major and trace element residences and mobilities from the raw shale determine the extent of incorporation of these elements in the spent shale matrix and ultimately the composition of leachate generated by the spent shale solid: water interaction. In order to understand leachate compositions, it is necessary to determine this water: solid interaction, but the solid wastes being considered are a dependent function of the raw shale material and the process parameters. Thus, in order to understand the chemical principles operative in leachate generation, it is necessary to elucidate the interplay of the raw material and the process parameters in the formation of the waste and then the interaction of the waste form with water. The leachate and the solid waste are dependent variables, while the raw shale and the process parameters are independent variables. These considerations are illustrated by results of chemical characterization and experimental studies of field generated spent shales. Results from field generated materials are used to describe important considerations relative to the understanding of leachate chemistry.

Peterson, E.J.; Wagner, P.

1982-01-01

57

Chemical Reactions on Ice Surfaces; Experimental and Theoretical Studies  

Microsoft Academic Search

Surfaces of ice particles in the stratosphere and troposphere are now widely recognized to heterogeneously catalyze chemical reactions that lead to changes in the chemical composition of the atmosphere, however, not much is known about the reaction mechanisms, their time scales, or why ice is required for the reactions. We address these issues with respect to heterogeneous chlorine activation reactions

Franz Geiger

2002-01-01

58

Computed Potential Energy Surfaces for Chemical Reactions  

NASA Technical Reports Server (NTRS)

A manuscript describing the calculations on the (1)CH2 + H2O, H2 + HCOH, and H2 + H2CO product channels in the CH3 + OH reaction, which were described in the last progress report, has been accepted for publication in J. Chem. Phys., and a copy of the manuscript is included in the appendix. The production of (1)CH2 in this reaction is important in hydrocarbon combustion since (1)CH2 is highly reactive and would be expected to insert into N2, possibly leading to a new source for prompt NO(x) (vide infra). During the last six months new calculations have been carried out for the NH2 + NO system, which is important in the thermal de-NO(x) process.

Heinemann, K.; Walch, Stephen P.; Levin, Eugene

1993-01-01

59

Nonlinear magnetoacoustic wave propagation with chemical reactions  

NASA Astrophysics Data System (ADS)

The magnetoacoustic problem with an application to sound wave propagation through electrically conducting fluids such as the ocean in the Earth's magnetic field, liquid metals, or plasmas has been addressed taking into account several simultaneous chemical reactions. Using continuum balance equations for the total mass, linear momentum, energy; as well as Maxwell's electrodynamic equations, a nonlinear beam equation has been developed to generalize the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation for a fluid with linear viscosity but nonlinear and diffraction effects. Thermodynamic parameters are used and not tailored to only an adiabatic fluid case. The chemical kinetic equations build on a relaxing media approach presented, for example, by K. Naugolnukh and L. Ostrovsky [Nonlinear Wave Processes in Acoustics (Cambridge Univ. Press, Cambridge, 1998)] for a linearized single reaction and thermodynamic pressure equation of state. Approximations for large and small relaxation times and for magnetohydrodynamic parameters [Korsunskii, Sov. Phys. Acoust. 36 (1990)] are examined. Additionally, Cattaneo's equation for heat conduction and its generalization for a memory process rather than a Fourier's law are taken into account. It was introduced for the heat flux depends on the temperature gradient at an earlier time to generate heat pulses of finite speed.

Margulies, Timothy Scott

2002-11-01

60

Chemical reaction fouling model for single-phase heat transfer  

SciTech Connect

A fouling model was developed on the premise that the chemical reaction for generation of precursor can take place in the bulk fluid, in the thermalboundary layer, or at the fluid/wall interface, depending upon the interactive effects of flu id dynamics, heat and mass transfer, and the controlling chemical reaction. The analysis was used to examine the experimental data for fouling deposition of polyperoxides produced by autoxidation of indene in kerosene. The effects of fluid and wall temperatures for two flow geometries were analyzed. The results showed that the relative effects of physical parameters on the fouling rate would differ for the three fouling mechanisms; therefore, it is important to identify the controlling mechanism in applying the closed-flow-loop data to industrial conditions.

Panchal, C.B. [Argonne National Lab., IL (United States); Watkinson, A.P. [British Columbia Univ., Vancouver, BC (Canada)

1993-08-01

61

GREEN CHEMICAL SYNTHESIS THROUGH CATALYSIS AND ALTERNATE REACTION CONDITIONS  

EPA Science Inventory

Green chemical synthesis through catalysis and alternate reaction conditions Encompassing green chemistry techniques and methodologies, we have initiated several projects at the National Risk Management Research laboratory that focus on the design and development of chemic...

62

Crossed molecular beam studies of atmospheric chemical reaction dynamics  

SciTech Connect

The dynamics of several elementary chemical reactions that are important in atmospheric chemistry are investigated. The reactive scattering of ground state chlorine or bromine atoms with ozone molecules and ground state chlorine atoms with nitrogen dioxide molecules is studied using a crossed molecular beams apparatus with a rotatable mass spectrometer detector. The Cl + O{sub 3} {yields} ClO + O{sub 2} reaction has been studied at four collision energies ranging from 6 kcal/mole to 32 kcal/mole. The derived product center-of-mass angular and translational energy distributions show that the reaction has a direct reaction mechanism and that there is a strong repulsion on the exit channel. The ClO product is sideways and forward scattered with respect to the Cl atom, and the translational energy release is large. The Cl atom is most likely to attack the terminal oxygen atom of the ozone molecule. The Br + O{sub 3} {yields} ClO + O{sub 2} reaction has been studied at five collision energies ranging from 5 kcal/mole to 26 kcal/mole. The derived product center-of-mass angular and translational energy distributions are quite similar to those in the Cl + O{sub 3} reaction. The Br + O{sub 3} reaction has a direct reaction mechanism similar to that of the Cl + O{sub 3} reaction. The electronic structure of the ozone molecule seems to play the central role in determining the reaction mechanism in atomic radical reactions with the ozone molecule. The Cl + NO{sub 2} {yields} ClO + NO reaction has been studied at three collision energies ranging from 10.6 kcal/mole to 22.4 kcal/mole. The center-of-mass angular distribution has some forward-backward symmetry, and the product translational energy release is quite large. The reaction proceeds through a short-lived complex whose lifetime is less than one rotational period. The experimental results seem to show that the Cl atom mainly attacks the oxygen atom instead of the nitrogen atom of the NO{sub 2} molecule.

Zhang, Jingsong

1993-04-01

63

Heterogeneous chemical reactions: Preparation of monodisperse latexes  

NASA Technical Reports Server (NTRS)

It is demonstrated that a photoinitiated emulsion polymerization can be carried out to a significant conversion in a SPAR rocket prototype polymerization vessel within the six minutes allowed for the experiment. The percentage of conversion was determined by both dilatometry and gravimetric methods with good agreement. The experimental results lead to the following conclusions: (1) emulsion polymerizations can be carried out to conversions as high as 75%, using a stable micellized styrene-SLS system plus photoinitiator; (2) dilatometry can be used to accurately determine both the rate and conversion of polymerization; (3) thermal expansion due to the light source and heat of reaction is small and can be corrected for if necessary; (4) although seeded emulsion polymerizations are unfavorable in photoinitiation, as opposed to chemical initiation, polymerizations can be carried out to at least 15% conversion using 7940A seed particles, with 0.05% solids; and (5) photoinitiation should be used to initiate polymerization in the SPAR rocket experiments because of the mechanical simplicity of the experiment.

Vanderhoff, J. W.; Micale, F. J.; El-Aasser, M. S.; Sterk, A. A.; Bethke, G. W.

1977-01-01

64

Bimolecular chemical reactions on weighted complex networks.  

PubMed

We investigate the kinetics of bimolecular chemical reactions A+A?0 and A+B?0 on weighted scale-free networks (WSFNs) with degree distribution P(k)?k^{-?} . On WSFNs, a weight w{ij} is assigned to the link between node i and j . We consider the symmetric weight given as w{ij}=(k{i}k{j})^{?} , where k{i} and k{j} are the degree of node i and j . The hopping probability T{ij} of a particle from node i to j is then given as T{ij}?(k{i}k{j})^{?} . From a mean-field analysis, we analytically show in the thermodynamic limit that the kinetics of A+A?0 and A+B?0 are identical and there exist two crossover ? values, ?{1c}=?-2 and ?{2c}=(?-3)/2 . The density of particles ?(t) algebraically decays in time t as t^{-?} with ?=1 for ?

Kwon, Sungchul; Choi, Woosik; Kim, Yup

2010-08-01

65

Laboratory Studies of Homogeneous and Heterogeneous Chemical Processes of Importance in the Upper Atmosphere  

NASA Technical Reports Server (NTRS)

The objective of this study was to conduct measurements of chemical kinetics parameters for reactions of importance in the stratosphere and upper troposphere, and to study the interaction of trace gases with ice surfaces in order to elucidate the mechanism of heterogeneous chlorine activation processes, using both a theoretical and an experimental approach. The measurements were carried out under temperature and pressure conditions covering those applicable to the stratosphere and upper troposphere. The main experimental technique employed was turbulent flow-chemical ionization mass spectrometry, which is particularly well suited for investigations of radical-radical reactions.

Molina, Mario J.

2003-01-01

66

Investigating Factors Influencing Rates of Chemical Reactions  

NSDL National Science Digital Library

This activity is a lab investigation in which students observe the rate of generation of hydrogen gas from a reaction, and then modify the procedure to compare another variable affecting the rate of this reaction.

Derickson, Paula

67

Semiclassical methods in chemical reaction dynamics  

SciTech Connect

Semiclassical approximations, simple as well as rigorous, are formulated in order to be able to describe gas phase chemical reactions in large systems. We formulate a simple but accurate semiclassical model for incorporating multidimensional tunneling in classical trajectory simulations. This model is based on the existence of locally conserved actions around the saddle point region on a multidimensional potential energy surface. Using classical perturbation theory and monitoring the imaginary action as a function of time along a classical trajectory we calculate state-specific unimolecular decay rates for a model two dimensional potential with coupling. Results are in good comparison with exact quantum results for the potential over a wide range of coupling constants. We propose a new semiclassical hybrid method to calculate state-to-state S-matrix elements for bimolecular reactive scattering. The accuracy of the Van Vleck-Gutzwiller propagator and the short time dynamics of the system make this method self-consistent and accurate. We also go beyond the stationary phase approximation by doing the resulting integrals exactly (numerically). As a result, classically forbidden probabilties are calculated with purely real time classical trajectories within this approach. Application to the one dimensional Eckart barrier demonstrates the accuracy of this approach. Successful application of the semiclassical hybrid approach to collinear reactive scattering is prevented by the phenomenon of chaotic scattering. The modified Filinov approach to evaluating the integrals is discussed, but application to collinear systems requires a more careful analysis. In three and higher dimensional scattering systems, chaotic scattering is suppressed and hence the accuracy and usefulness of the semiclassical method should be tested for such systems.

Keshavamurthy, S.

1994-12-01

68

Chemical kinetics computer program for static and flow reactions  

NASA Technical Reports Server (NTRS)

General chemical kinetics computer program for complex gas mixtures has been developed. Program can be used for any homogeneous reaction in either one dimensional flow or static system. It is flexible, accurate, and easy to use. It can be used for any chemical system for which species thermodynamic data and reaction rate constant data are known.

Bittker, D. A.; Scullin, V. J.

1972-01-01

69

Heat-of-Reaction Chemical Heat Pumps.  

National Technical Information Service (NTIS)

Chemical heat pumps are mechanically driven heat pumps with working fluids that undergo chemical changes or are heat-driven heat pumps in which either the driver (heat engine) or heat pump utilizes a reactive working fluid. As such, chemical heat pumps ca...

C. Bliem L. Kirol

1988-01-01

70

Mechano-chemical coupling in Belousov-Zhabotinskii reactions  

NASA Astrophysics Data System (ADS)

Mechano-chemical coupling has been recently recognised as an important effect in various systems as chemical reactivity can be controlled through an applied mechanical loading. Namely, Belousov-Zhabotinskii reactions in polymer gels exhibit self-sustained oscillations and have been identified to be reasonably controllable and definable to the extent that they can be harnessed to perform mechanical work at specific locations. In this paper, we use our theoretical work of nonlinear mechano-chemical coupling and investigate the possibility of providing an explanation of phenomena found in experimental research by means of this theory. We show that mechanotransduction occurs as a response to both static and dynamic mechanical stimulation, e.g., volume change and its rate, as observed experimentally and discuss the difference of their effects on oscillations. Plausible values of the quasi-stoichiometric parameter f of Oregonator model are estimated together with its dependence on mechanical stimulation. An increase in static loading, e.g., pressure, is predicted to have stimulatory effect whereas dynamic loading, e.g., rate of volume change, is predicted to be stimulatory only up to a certain threshold. Further, we offer a physically consistent explanation of the observed phenomena why some Belousov-Zhabotinskii gels require an additional mechanical stimulation to show emergence of oscillation or why "revival" of oscillations in Belousov-Zhabotinskii reactions is possible together with indications for further experimental setups.

Klika, Václav; Grmela, Miroslav

2014-03-01

71

Chemical Demonstrations with Consumer Chemicals: The Black and White Reaction.  

ERIC Educational Resources Information Center

Describes a dramatic chemical demonstration in which chemicals that are black and white combine to produce a colorless liquid. Reactants include tincture of iodine, bleach, white vinegar, Epsom salt, vitamin C tablets, and liquid laundry starch. (DDR)

Wright, Stephen W.

2002-01-01

72

Test of Weak Reaction Rates of Importance for Late Stellar Evolution Using Charge-exchange Reactions.  

NASA Astrophysics Data System (ADS)

Weak reactions (electron captures and beta decays) on nuclei play an important role in the evolution of pre-supernovae stars and their eventual core collapse. The rates are usually predicted in shell-model and mean-field calculations. Experimental information on Gamow-Teller strength distributions is needed to test these calculations. At the NSCL, the (t,^3He) charge-exchange reaction at 115 MeV/nucleon is employed for such studies. In addition, the (^3He,t) reaction at 140 MeV/nucleon (at RCNP, Osaka) is used for detailed studies of the charge-exchange reaction mechanism, which is important for understanding the uncertainties in the experimentally extracted Gamow-Teller strength distributions. Besides a comparison between experimental and theoretical Gamow-Teller strength distributions, the work also includes the calculation of electron-capture rates, so that the sensitivity of the rates on differences between theoretical and experimental rates can be gauged. A significant fraction of the nuclei relevant for late stellar evolution are unstable. Hence, to ensure accurate weak reaction rates for such nuclei, charge-exchange experiments on rare isotopes are crucial, but the techniques have to be developed. At the NSCL, a program has been initiated to study charge-exchange reactions on rare isotopes.

Zegers, R. G. T.

2009-05-01

73

Chemical Behaviour of Pu and Am: Hydrolysis Reaction in Brine Solutions, Carbonate Complexation, alpha -Radiolysis, Humate Complexation and Speciation.  

National Technical Information Service (NTIS)

The chemical behaviour of transuranic elements (Pu and Am) has been investigated in saline solution of different NaCl concentrations in the near neutral pH range. Important reactions considered are hydrolysis, carbonate complexation, redox reaction, alpha...

J. I. Kim G. Buckau K. Bueppelmann R. Klenze C. Lierse

1986-01-01

74

Systems of Chemical Equations as Reasonable Reaction Mechanisms  

NASA Astrophysics Data System (ADS)

This paper demonstrates that chemical equations may be operated like a kind of LEGO game, with construction of the systems of chemical equations. In my teaching experience, these systems of chemical equations are able to help students to understand the reaction routes. Six general principles of creating the systems are formulated. Three examples from inorganic chemistry are considered and discussed in detail.

Dorozhkin, Sergey V.

2001-07-01

75

A Case Study in Chemical Kinetics: The OH + CO Reaction.  

ERIC Educational Resources Information Center

Reviews some important properties of the bimolecular reaction between the hydroxyl radical and carbon monoxide. Investigates the kinetics of the reaction, the temperature and pressure dependence of the rate constant, the state-to-state dynamics of the reaction, and the reverse reaction. (MVL)

Weston, Ralph E., Jr.

1988-01-01

76

FACILITATED CHEMICAL SYNTHESIS UNDER ALTERNATE REACTION CONDITIONS  

EPA Science Inventory

The chemical research in the late 1990's witnessed a paradigm shift towards "environmentally-friendly chemistry" more popularly known as "green chemistry" due to the increasing environmental concerns and legislative requirements to curb the release of chemical waste into the atmo...

77

The Mystery Reaction: A Lesson on Chemical Reactions  

NSDL National Science Digital Library

This teaching resource was developed by a K-12 science teacher in the American Physiologycal Society's 2006 Frontiers in Physiology Program. For more information on this program, please visit www.frontiersinphys.org. The purpose of this lesson is to design an investigation and conduct an experiment that will allow students to explore the differences between physical and chemical changes. In this investigation, they are given the opportunity to develop a list of evidence for determining whether or not a chemical change has occurred.

Tonya Williams (Kelly Miller Middle School)

2006-08-01

78

Kinetics of Chemical Reactions in Flames  

NASA Technical Reports Server (NTRS)

In part I of the paper the theory of flame propagation is developed along the lines followed by Frank-Kamenetsky and one of the writers. The development of chain processes in flames is considered. A basis is given for the application of the method of stationary concentrations to reactions in flames; reactions with branching chains are analyzed. The case of a diffusion coefficient different from the coefficient of temperature conductivity is considered.

Zeldovich, Y.; Semenov, N.

1946-01-01

79

Developing Secondary Students' Conceptions of Chemical Reactions: The Introduction of Chemical Equilibrium.  

ERIC Educational Resources Information Center

Describes an empirical study concerning the introduction of the concept of chemical equilibrium in chemistry classrooms in a way which challenges students' initial conceptions of chemical reactions. Contains 23 references. (DDR)

Van Driel, Jan H.; De Vos, Wobbe; Verloop, Nico; Dekkers, Hetty

1998-01-01

80

Computed potential energy surfaces for chemical reactions  

NASA Technical Reports Server (NTRS)

The work on the NH + NO system which was described in the last progress report was written up and a draft of the manuscript is included in the appendix. The appendix also contains a draft of a manuscript on an Ar + H + H surface. New work which was completed in the last six months includes the following: (1) calculations on the (1)CH2 + H2O, H2 + HCOH, and H2 + H2CO product channels in the CH3 + OH reaction; (2) calculations for the NH2 + O reaction; (3) calculations for the CH3 + O2 reaction; and (4) calculations for CH3O and the two decomposition channels--CH2OH and H + H2CO. Detailed descriptions of this work will be given in manuscripts; however, brief descriptions of the CH3 + OH and CH3 + O2 projects are given.

Heinemann, K.; Walch, Stephen P.

1992-01-01

81

Importance of plasma membrane dynamics in chemical-induced carcinogenesis.  

PubMed

In the last decade, a lot of patents have been filled regarding molecular biology and functions of cellular membranes. The membrane bilayer model has evolved from a static, passive, homogeneous barrier to a highly dynamic, asymmetric, heterogeneous structure composed of distinct domains. Changes in membrane fluidity and composition of microdomains have been proven to be involved in the regulation of many important physiological signaling pathways. Recently, several xenobiotics, including various drugs and environmental pollutants, have been reported to change plasma membrane characteristics, thereby altering cell physiology. Interestingly, it has been suggested that a cross talk between chemical-induced cellular membrane effects and DNA damages may be important for the final mutation outcome of genotoxic chemicals. Thus, effects on plasma membrane remodeling may give additional mechanistic explanations to how certain chemicals exert their carcinogenic effect. With respect to such effects, recent patents suggest to focus on plasma membrane and its components like caveolin-1 for cancer screening and chemotherapy. Here, we review the effects of environmental toxicants on cellular plasma membrane structure and function, and further describe possible implication for health and disease. PMID:21756246

Tekpli, Xavier; Holme, Jørn A; Sergent, Odile; Lagadic-Gossmann, Dominique

2011-09-01

82

Parameter estimation in complex flows with chemical reactions  

NASA Astrophysics Data System (ADS)

The estimation of unknown parameters in engineering and scientific models continues to be of great importance in order to validate them to available experimental data. These parameters of concern cannot be known beforehand, but must be measured experimentally, variables such as chemical species concentrations, pressures, or temperatures as examples. Particularly, in chemically reacting flows, the estimation of kinetic rate parameters from experimentally determined values is in great demand and not well understood. New parameter optimization algorithms have been developed from a Gauss-Newton formulation for the estimation of reaction rate parameters in several different complex flow applications. A zero-dimensional parameter estimation methodology was used in conjunction with a parameter sensitivity study and then applied to three-dimensional flow models. This new parameter estimation technique was applied to three-dimensional models for chemical vapor deposition of silicon carbide and gallium arsenide semiconductor materials. The parameter estimation for silicon carbide for several different operating points was in close agreement to experiment. The parameter estimation for gallium arsenide proved to be very accurate, being within four percent of the experimental data. New parameter estimation algorithms were likewise created for a three-dimensional multiphase model for methanol spray combustion. The kinetic rate parameters delivered results in close agreement to experiment for profiles of combustion species products. In addition, a new parameter estimation method for the determination of spray droplet sizes and velocities is presented. The results for methanol combustion chemical species profiles are in good agreement to experiment for several different droplet sizes. Lastly, the parameter estimation method was extended to a bio-kinetic application, namely mitochondrial cells, that are cardiac or respiratory cells found in animals and humans. The results for the electrochemical parameters examined again show very close agreement to the experimental values. The usual method of determining chemical reaction rates by measurement is lengthy and time consuming, especially where the chemistry and chemical processes are of a more complex nature. This computational method and associated algorithms has shown to deliver accurate correlations to the experimental data for three-dimensional models, demonstrated in these new unique applications of the technique.

Robinson, Daniel J.

83

Drying with Chemical Reaction in Cocoa Beans  

Microsoft Academic Search

Desirable flavor qualities of cocoa are dependent on how the cocoa beans are fermented, dried, and roasted. During fermentation and drying, polyphenols such as leucocyanidin and apecatechin are oxidized by polyphenols oxidase to form o-quinone, which later react nonenzymatically with a hydroquinone in a condensation reaction to form browning products and moisture. The objective of this article is to model

Wan Ramli Wan Daud; Meor Zainal Meor Talib; Tin Mar Kyi

2007-01-01

84

Potential energy surfaces for chemical reactions  

Microsoft Academic Search

The research completed spans a rather broad range of chemistry. Taken as a body it conforms well to our goal of three years ago to contribute in a significant way to the fundamental underpinnings of combustion chemistry. It includes the reaction of methane with molecular oxygen, research on protonated ethane and the hydronium ion, aluminum atom-unsaturated hydrocarbon systems, the infrared

1990-01-01

85

Laboratory studies of sticking coefficients and heterogeneous reactions important in the stratosphere  

NASA Technical Reports Server (NTRS)

The discovery of ozone depletion during spring in the Antarctic stratosphere has received wide spread attention. Both meteorological and chemical mechanisms have been used in attempts to explain this observation. The chemical theory focused on the chlorofluoromethanes released into the atmosphere. However, gas-phase, homogeneous reactions alone in the model can not adequately explain such a depletion. Heterogeneous reactions on ice surfaces could be important in the Antarctic stratosphere. These reactions are thought to convert ClONO2 and N2O5 into HNO3 in the solid phase while Cl2, HOCl, and ClNO2 are released into the stratosphere as gas-phase products. The photolysis of Cl2, HOCl, and ClNO2, then produces active chlorine which subsequently removes ozone through several catalytic cycles, including the Cl2O2 mechanism. The polar stratospheric clouds are thought to consist of mixtures of water ice, nitric acid, and sulfuric acid. Condensation of HCl onto the PSC's could provide active surfaces for heterogeneous reactions.

Leu, Ming-Taun

1988-01-01

86

Electronic excitations by chemical reactions on metal surfaces  

Microsoft Academic Search

Dissipation of chemical energy released in exothermic reactions at metal surfaces may happen adiabatically by creation of phonons or non-adiabatically by excitation of the electronic system of the metal or the reactants. In the past decades, the only direct experimental evidence for such non-adiabatic reactions has been exoelectron emission into vacuum and surface chemiluminescence which are observed in a special

Hermann Nienhaus

2002-01-01

87

Evaluated Chemical Kinetic Rate Constants for Various Gas Phase Reactions  

Microsoft Academic Search

The available information, up to mid-1972, for the rate constants of a series of gas phase chemical reactions has been evaluated critically. For each reaction, relevant thermodynamic data are presented and values for the equilibrium constant expressed in mathematical form. Kinetic data are presented in tabular and graphical form together with a discussion of the pertinent details. Recommended rate constant

Keith Schofield

1973-01-01

88

Cu-free click cycloaddition reactions in chemical biology†  

PubMed Central

Bioorthogonal chemical reactions are paving the way for new innovations in biology. These reactions possess extreme selectivity and biocompatibility, such that their participating reagents can form covalent bonds within richly functionalized biological systems—in some cases, living organisms. This tutorial review will summarize the history of this emerging field, as well as recent progress in the development and application of bioorthogonal copper-free click cycloaddition reactions.

Jewett, John C.

2010-01-01

89

Results of the 2010 Survey on Teaching Chemical Reaction Engineering  

ERIC Educational Resources Information Center

A survey of faculty teaching the chemical reaction engineering course or sequence during the 2009-2010 academic year at chemical engineering programs in the United States and Canada reveals change in terms of content, timing, and approaches to teaching. The report consists of two parts: first, a statistical and demographic characterization of the…

Silverstein, David L.; Vigeant, Margot A. S.

2012-01-01

90

Laser cutting with chemical reaction assist  

DOEpatents

A method for cutting with a laser beam where an oxygen-hydrocarbon reaction is used to provide auxiliary energy to a metal workpiece to supplement the energy supplied by the laser. Oxygen is supplied to the laser focus point on the workpiece by a nozzle through which the laser beam also passes. A liquid hydrocarbon is supplied by coating the workpiece along the cutting path with the hydrocarbon prior to laser irradiation or by spraying a stream of hydrocarbon through a nozzle aimed at a point on the cutting path which is just ahead of the focus point during irradiation.

Gettemy, Donald J. (Los Alamos, NM) [Los Alamos, NM

1992-01-01

91

Laser cutting with chemical reaction assist  

DOEpatents

A method is described for cutting with a laser beam where an oxygen-hydrocarbon reaction is used to provide auxiliary energy to a metal workpiece to supplement the energy supplied by the laser. Oxygen is supplied to the laser focus point on the workpiece by a nozzle through which the laser beam also passes. A liquid hydrocarbon is supplied by coating the workpiece along the cutting path with the hydrocarbon prior to laser irradiation or by spraying a stream of hydrocarbon through a nozzle aimed at a point on the cutting path which is just ahead of the focus point during irradiation. 1 figure.

Gettemy, D.J.

1992-11-17

92

LIGAND: Database of Chemical Compounds and Reactions in Biological Pathways  

NSDL National Science Digital Library

The Institute for Chemical Research at Kyoto University provides this frequently updated and well-documented database of enzyme reactions. With more than 9,300 entries, the LIGAND Chemical Database includes over 3,700 entries for enzymes (the Enzyme Reaction Database) and 5,600 entries for compounds (Chemical Compound Database). The database is searchable by keyword using DBGET (which supports numerous other databases and gene catalogs as well) and is accompanied by clear instructions. The LIGAND database, updated weekly, may be downloaded via anonymous FTP.

93

A robustness screen for the rapid assessment of chemical reactions  

NASA Astrophysics Data System (ADS)

In contrast to the rapidity with which scientific information is published, the application of new knowledge often remains slow, and we believe this to be particularly true of newly developed synthetic organic chemistry methodology. Consequently, methods to assess and identify robust chemical reactions are desirable, and would directly facilitate the application of newly reported synthetic methodology to complex synthetic problems. Here, we describe a simple process for assessing the likely scope and limitations of a chemical reaction beyond the idealized reaction conditions initially reported. Using simple methods and common analytical techniques we demonstrate a rapid assessment of an established chemical reaction, and also propose a simplified analysis that may be reported alongside new synthetic methodology.

Collins, Karl D.; Glorius, Frank

2013-07-01

94

Gas-phase reactions of halogen species of atmospheric importance  

NASA Astrophysics Data System (ADS)

A low-pressure discharge-flow technique, with various optical detection methods, has been used to determine bimolecular rate coefficients for a number of reactions in the gas-phase between OH radicals and organic halogen-containing molecules and between NO3 radicals and the iodine species I2 and I. These experiments have shown that: (1) the reaction of methyl iodide with OH accounts for approximately 2 percent of the removal of CH3I from the troposphere as compared with photolysis; (2) abstraction of I-atoms from a C-I bond by OH is probable in the gas-phase; (3) the halogen-containing anaesthetic substances halothane CF3CClBrH, enflurane CF2HOCF2CFClH, isoflurane CF2HOCClHCF3 and sevoflurane (CF3)2CHOCFH2 have significantly shorter tropospheric lifetimes than the fully halogenated CFCs and halons because of reaction with the OH radical and are thus unlikely to be transported up to the stratosphere where they could contribute to the depletion of ozone. Data obtained for reactions between OH and some 'CFC alternatives' along with measurements of the integrated absorption cross-sections of the compounds in the spectral region 800-1200 cm(exp -1) were used to calculate ozone depletion potentials (ODP) and greenhouse warming potentials relative to CFCl3 for each compound. The study of the reactions between OH and CF3CFBrH and CF2BrH was used to provide a useful first estimate of the environmental acceptability of these compounds in the context of their possible use as replacements for the conventional CFCs. A method was developed to provide a first estimate of the ODP of a halogenated alkane without use of a complicated (and expensive) computer modeling scheme. A reaction between molecular iodine and the nitrate radical in the gas-phase was discovered and the kinetics of this reaction have been studied. No temperature or pressure dependence was observed for the rate of reaction, the rate constant of which was found to be (1.5 +/- 0.5) x 10(exp -12)/cu cm/molecule/s. The reaction between I and NO3 was found to occur at a rate of about 60 percent of the hard-sphere collision frequency for the two species. The rate constant for reaction between I and NO3 was found to be (4.5 +/- 1.9) x 10(exp -10)/cu cm/molecule/s. An upper limit for the heat of formation of IONO2 of (21 +/- 3) kJmol(exp -1) was also derived.

Heard, Anne C.

95

Analysis of the Reaction Rate Coefficients for Slow Bimolecular Chemical Reactions  

NASA Astrophysics Data System (ADS)

Simple bimolecular reactions A_1+A_2A_3+A_4 are analyzed within the framework of the Boltzmann equation in the initial stage of a chemical reaction with the system far from chemical equilibrium. The Chapman-Enskog methodology is applied to determine the coefficients of the expansion of the distribution functions in terms of Sonine polynomials for peculiar molecular velocities. The results are applied to the reaction H_2 +ClHCl+H, and the influence of the non-Maxwellian distribution and of the activation-energy dependent reactive cross sections upon the forward and reverse reaction rate coefficients are discussed.

Kremer, Gilberto M.; Silva, Tiago G.

2012-12-01

96

Single-molecule chemical reaction reveals molecular reaction kinetics and dynamics.  

PubMed

Understanding the microscopic elementary process of chemical reactions, especially in condensed phase, is highly desirable for improvement of efficiencies in industrial chemical processes. Here we show an approach to gaining new insights into elementary reactions in condensed phase by combining quantum chemical calculations with a single-molecule analysis. Elementary chemical reactions in liquid-phase, revealed from quantum chemical calculations, are studied by tracking the fluorescence of single dye molecules undergoing a reversible redox process. Statistical analyses of single-molecule trajectories reveal molecular reaction kinetics and dynamics of elementary reactions. The reactivity dynamic fluctuations of single molecules are evidenced and probably arise from either or both of the low-frequency approach of the molecule to the internal surface of the SiO2 nanosphere or the molecule diffusion-induced memory effect. This new approach could be applied to other chemical reactions in liquid phase to gain more insight into their molecular reaction kinetics and the dynamics of elementary steps. PMID:24963600

Zhang, Yuwei; Song, Ping; Fu, Qiang; Ruan, Mingbo; Xu, Weilin

2014-01-01

97

Threshold resonances in ultracold chemical reactions  

NASA Astrophysics Data System (ADS)

We analyze the effects of near threshold resonances on the low energy behavior of cross sections for reactive scattering systems with reaction a barrier (e.g. Cl + H2, D + H2). We find an anomalous behavior when a resonance pole is very close to the threshold of the entrance channel. For inelastic processes, including reactive ones, the anomalous energy dependence of the cross sections is given by ?˜E-3/2. However, at vanishingly low energies, the standard Wigner's threshold behavior (?˜E-1/2) is eventually recovered, but limiting to much narrower range of energies. When the cross sections are averaged to obtain rate coefficients, the anomalous behavior persists; indeed, we find an intermediate regime of ultralow temperatures, where the inelastic rate coefficients behave as K˜1/T.

Côté, Robin; Simbotin, Ionel; Ghosal, Subhas

2012-06-01

98

Imaging chemical reactions - 3D velocity mapping  

NASA Astrophysics Data System (ADS)

Visualising a collision between an atom or a molecule or a photodissociation (half-collision) of a molecule on a single particle and single quantum level is like watching the collision of billiard balls on a pool table: Molecular beams or monoenergetic photodissociation products provide the colliding reactants at controlled velocity before the reaction products velocity is imaged directly with an elaborate camera system, where one should keep in mind that velocity is, in general, a three-dimensional (3D) vectorial property which combines scattering angles and speed. If the processes under study have no cylindrical symmetry, then only this 3D product velocity vector contains the full information of the elementary process under study.

Chichinin, A. I.; Gericke, K.-H.; Kauczok, S.; Maul, C.

99

Automatic NMR-Based Identification of Chemical Reaction Types in Mixtures of Co-Occurring Reactions  

PubMed Central

The combination of chemoinformatics approaches with NMR techniques and the increasing availability of data allow the resolution of problems far beyond the original application of NMR in structure elucidation/verification. The diversity of applications can range from process monitoring, metabolic profiling, authentication of products, to quality control. An application related to the automatic analysis of complex mixtures concerns mixtures of chemical reactions. We encoded mixtures of chemical reactions with the difference between the 1H NMR spectra of the products and the reactants. All the signals arising from all the reactants of the co-occurring reactions were taken together (a simulated spectrum of the mixture of reactants) and the same was done for products. The difference spectrum is taken as the representation of the mixture of chemical reactions. A data set of 181 chemical reactions was used, each reaction manually assigned to one of 6 types. From this dataset, we simulated mixtures where two reactions of different types would occur simultaneously. Automatic learning methods were trained to classify the reactions occurring in a mixture from the 1H NMR-based descriptor of the mixture. Unsupervised learning methods (self-organizing maps) produced a reasonable clustering of the mixtures by reaction type, and allowed the correct classification of 80% and 63% of the mixtures in two independent test sets of different similarity to the training set. With random forests (RF), the percentage of correct classifications was increased to 99% and 80% for the same test sets. The RF probability associated to the predictions yielded a robust indication of their reliability. This study demonstrates the possibility of applying machine learning methods to automatically identify types of co-occurring chemical reactions from NMR data. Using no explicit structural information about the reactions participants, reaction elucidation is performed without structure elucidation of the molecules in the mixtures.

Latino, Diogo A. R. S.; Aires-de-Sousa, Joao

2014-01-01

100

Gas-Phase Reactions of Halogen Species of Atmospheric Importance.  

NASA Astrophysics Data System (ADS)

Available from UMI in association with The British Library. Requires signed TDF. A low-pressure discharge-flow technique, with various optical detection methods, has been used to determine bimolecular rate coefficients for a number of reactions in the gas-phase between OH radicals and organic halogen -containing molecules and between NO_3 radicals and the iodine species I_2 and I. These experiments have shown that: (i) the reaction of methyl iodide with OH accounts for approximately 2% of the removal of CH_3I from the troposphere as compared with photolysis; (ii) abstraction of I-atoms from a C-I bond by OH is probable in the gas -phase; (iii) the halogen-containing anaesthetic substances halothane CF_3CCl BrH, enflurane CF_2HOCF _2CFClH, isoflurane CF_2HOCClHCF _3 and sevoflurane (CF_3) _2CHOCFH_2 have significantly shorter tropospheric lifetimes than the fully halogenated CFCs and halons because of reaction with the OH radical and are thus unlikely to be transported up to the stratosphere where they could contribute to the depletion of ozone. Data obtained for reactions between OH and some 'CFC alternatives' along with measurements of the integrated absorption cross -sections of the compounds in the spectral region 800-1200 cm^{-1} were used to calculate ozone depletion potentials (ODP) and greenhouse warming potentials relative to CFCl_3 for each compound. The study of the reactions between OH and CF_3CFBrH and CF _2BrH was used to provide a useful first estimate of the environmental acceptability of these compounds in the context of their possible use as replacements for the conventional CFCs. A method was developed to provide a first estimate of the ODP of a halogenated alkane without use of a complicated (and expensive) computer modeling scheme. A reaction between molecular iodine and the nitrate radical in the gas-phase was discovered and the kinetics of this reaction have been studied. No temperature or pressure dependence was observed for the rate of reaction, the rate constant of which was found to be (1.5 +/- 0.5) times 10 ^{-12}cm^{ -3}molecule^{-1}s ^{-1}. The reaction between I and NO_3 was found to occur at a rate of about 60% of the hard-sphere collision frequency for the two species. The rate constant for reaction between I and NO_3 was found to be (4.5 +/- 1.9) times 10^{-10}cm^3 molecule^{-1}s ^{-1}. An upper limit for the heat of formation of IONO_2 of (21 +/- 3) kJmol^ {-1} was also derived. (Abstract shortened by UMI.).

Heard, Anne C.

101

Computed potential energy surfaces for chemical reactions  

NASA Technical Reports Server (NTRS)

The minimum energy path for the addition of a hydrogen atom to N2 is characterized in CASSCF/CCI calculations using the (4s3p2d1f/3s2p1d) basis set, with additional single point calculations at the stationary points of the potential energy surface using the (5s4p3d2f/4s3p2d) basis set. These calculations represent the most extensive set of ab initio calculations completed to date, yielding a zero point corrected barrier for HN2 dissociation of approx. 8.5 kcal mol/1. The lifetime of the HN2 species is estimated from the calculated geometries and energetics using both conventional Transition State Theory and a method which utilizes an Eckart barrier to compute one dimensional quantum mechanical tunneling effects. It is concluded that the lifetime of the HN2 species is very short, greatly limiting its role in both termolecular recombination reactions and combustion processes.

Walch, Stephen P.

1988-01-01

102

Mining chemical reactions using neighborhood behavior and condensed graphs of reactions approaches.  

PubMed

This work addresses the problem of similarity search and classification of chemical reactions using Neighborhood Behavior (NB) and Condensed Graphs of Reaction (CGR) approaches. The CGR formalism represents chemical reactions as a classical molecular graph with dynamic bonds, enabling descriptor calculations on this graph. Different types of the ISIDA fragment descriptors generated for CGRs in combination with two metrics--Tanimoto and Euclidean--were considered as chemical spaces, to serve for reaction dissimilarity scoring. The NB method has been used to select an optimal combination of descriptors which distinguish different types of chemical reactions in a database containing 8544 reactions of 9 classes. Relevance of NB analysis has been validated in generic (multiclass) similarity search and in clustering with Self-Organizing Maps (SOM). NB-compliant sets of descriptors were shown to display enhanced mapping propensities, allowing the construction of better Self-Organizing Maps and similarity searches (NB and classical similarity search criteria--AUC ROC--correlate at a level of 0.7). The analysis of the SOM clusters proved chemically meaningful CGR substructures representing specific reaction signatures. PMID:22894688

de Luca, Aurélie; Horvath, Dragos; Marcou, Gilles; Solov'ev, Vitaly; Varnek, Alexandre

2012-09-24

103

STM CONTROL OF CHEMICAL REACTIONS: Single-Molecule Synthesis  

NASA Astrophysics Data System (ADS)

The fascinating advances in single atom/molecule manipulation with a scanning tunneling microscope (STM) tip allow scientists to fabricate atomic-scale structures or to probe chemical and physical properties of matters at an atomic level. Owing to these advances, it has become possible for the basic chemical reaction steps, such as dissociation, diffusion, adsorption, readsorption, and bond-formation processes, to be performed by using the STM tip. Complete sequences of chemical reactions are able to induce at a single-molecule level. New molecules can be constructed from the basic molecular building blocks on a one-molecule-at-a-time basis by using a variety of STM manipulation schemes in a systematic step-by-step manner. These achievements open up entirely new opportunities in nanochemistry and nanochemical technology. In this review, various STM manipulation techniques useful in the single-molecule reaction process are reviewed, and their impact on the future of nanoscience and technology are discussed.

Hla, Saw-Wai; Rieder, Karl-Heinz

2003-10-01

104

Asymmetric chemical reactions by polarized quantum beams  

NASA Astrophysics Data System (ADS)

One of the most attractive hypothesis for the origin of homochirality in terrestrial bio-organic compounds (L-amino acid and D-sugar dominant) is nominated as "Cosmic Scenario"; a chiral impulse from asymmetric excitation sources in space triggered asymmetric reactions on the surfaces of such space materials as meteorites or interstellar dusts prior to the existence of terrestrial life. 1) Effective asymmetric excitation sources in space are proposed as polarized quantum beams, such as circularly polarized light and spin polarized electrons. Circularly polarized light is emitted as synchrotron radiation from tightly captured electrons by intense magnetic field around neutron stars. In this case, either left-or right-handed polarized light can be observed depending on the direction of observation. On the other hand, spin polarized electrons is emitted as beta-ray in beta decay from radioactive nuclei or neutron fireballs in supernova explosion. 2) The spin of beta-ray electrons is longitudinally polarized due to parity non-conservation in the weak interaction. The helicity (the the projection of the spin onto the direction of kinetic momentum) of beta-ray electrons is universally negative (left-handed). For the purpose of verifying the asymmetric structure emergence in bio-organic compounds by polarized quantum beams, we are now carrying out laboratory simulations using circularly polarized light from synchrotron radiation facility or spin polarized electron beam from beta-ray radiation source. 3,4) The target samples are solid film or aqueous solution of racemic amino acids. 1) K.Kobayashi, K.Kaneko, J.Takahashi, Y.Takano, in Astrobiology: from simple molecules to primitive life; Ed. V.Basiuk; American Scientific Publisher: Valencia, 2008. 2) G.A.Gusev, T.Saito, V.A.Tsarev, A.V.Uryson, Origins Life Evol. Biosphere. 37, 259 (2007). 3) J.Takahashi, H.Shinojima, M.Seyama, Y.Ueno, T.Kaneko, K.Kobayashi, H.Mita, M.Adachi, M.Hosaka, M.Katoh, Int. J. Mol. Sci. 10, 3044 (2009). 4) V.I.Burkov, L.A.Goncharova, G.A.Gusev, H.Hashimoto, F.Kaneko, T.Kaneko, K. Kobayashi, H.Mita, E.V.Moiseenko, T.Ogawa, N.G.Poluhina, T.Saito, S.Shima, J.Takahashi, M.Tanaka, Y.Tao, V.A.Tsarev, J.Xu, H.Yabuta, K.Yagi-Watanabe, H.Yan, G.Zhang, Origins Life Evol. Biosphere, 39 295 (2009).

Takahashi, Jun-Ichi; Kobayashi, Kensei

105

Maximum Probability Reaction Sequences in Stochastic Chemical Kinetic Systems  

PubMed Central

The detailed behavior of many molecular processes in the cell, such as protein folding, protein complex assembly, and gene regulation, transcription and translation, can often be accurately captured by stochastic chemical kinetic models. We investigate a novel computational problem involving these models – that of finding the most-probable sequence of reactions that connects two or more states of the system observed at different times. We describe an efficient method for computing the probability of a given reaction sequence, but argue that computing most-probable reaction sequences is EXPSPACE-hard. We develop exact (exhaustive) and approximate algorithms for finding most-probable reaction sequences. We evaluate these methods on test problems relating to a recently-proposed stochastic model of folding of the Trp-cage peptide. Our results provide new computational tools for analyzing stochastic chemical models, and demonstrate their utility in illuminating the behavior of real-world systems.

Salehi, Maryam; Perkins, Theodore J.

2010-01-01

106

Phase Separation in Binary Mixtures with Chemical Reactions  

NASA Astrophysics Data System (ADS)

This paper is the second in a two-stage exposition on phase separation in binary (AB) mixtures with simple chemical reactions, which involve only the components A and B of the mixture. In this paper, we present details of our modelling, which employs the master equation formalism, for a number of different reactions. We also present detailed numerical results which supplement those in our previous paper (S. Puri and H. L. Frisch, J. Phys. A27, 6027 (1994)).

Puri, Sanjay; Frisch, Harry L.

107

Surface light-induced drift affected by chemical reactions  

Microsoft Academic Search

Surface light-induced drift caused by backscattering from rough surfaces can be strongly affected by surface-enhanced state-specific chemical reactions. Expressions for the drift fluxes of the incident resonant-gas component and of the reaction product are presented. A resulting asymmetry in the spatial distributions of the components in a ssT-shaped enclosure is discussed, as a possible sensitive tool for studying the state

M. A. Vaksman; A. Ben-Reuven

1992-01-01

108

Method and apparatus for controlling gas evolution from chemical reactions  

DOEpatents

The present invention is directed toward monitoring a thermally driven gas evolving chemical reaction with an acoustic apparatus. Signals from the acoustic apparatus are used to control a heater to prevent a run-away condition. A digestion module in combination with a robotic arm further automate physical handling of sample material reaction vessels. The invention is especially useful for carrying out sample procedures defined in EPA Methods SW-846. 8 figs.

Skorpik, J.R.; Dodson, M.G.

1999-05-25

109

Method and apparatus for controlling gas evolution from chemical reactions  

DOEpatents

The present invention is directed toward monitoring a thermally driven gas evolving chemical reaction with an acoustic apparatus. Signals from the acoustic apparatus are used to control a heater to prevent a run-away condition. A digestion module in combination with a robotic arm further automate physical handling of sample material reaction vessels. The invention is especially useful for carrying out sample procedures defined in EPA Methods SW-846.

Skorpik, James R. (Kennewick, WA); Dodson, Michael G. (Richland, WA)

1999-01-01

110

Mass transfer and chemical reactions in reactive deformable bubble swarms  

NASA Astrophysics Data System (ADS)

A hybrid numerical/experimental technique was developed for the study of the impact of the multiphase hydrodynamics on mass transfer and chemical reactions at deformable interfaces. Different material properties and flow conditions can yield flows with qualitatively different mass transfer and transport characteristics. As many (bio-) reaction systems exhibit sensitivity to mass transport in general, and mixing specifically, it is possible to control their product distribution by tailoring the system parameters.

Koynov, Athanas; Tryggvason, Grétar; Schlüter, Michael; Khinast, Johannes G.

2006-03-01

111

Channel flow of a Maxwell fluid with chemical reaction  

NASA Astrophysics Data System (ADS)

This work is concerned with the two-dimensional boundary layer flow of an upper-convected Maxwell (UCM) fluid in a channel with chemical reaction. The walls of the channel are porous. Employing similarity transformations the governing non-linear partial differential equations are reduced into non-linear ordinary differential equations. The resulting ordinary differential equations are solved analytically using homotopy analysis method (HAM). Expressions for series solutions are derived. The convergence of the obtained series solutions are shown explicitly. The effects of Reynold’s number Re, Deborah number De, Schmidt number Sc and chemical reaction parameter ? on the velocity and the concentration fields are shown through graphs and discussed.

Hayat, T.; Abbas, Z.

2008-01-01

112

Chemical reactions in viscous liquids under space conditions  

NASA Astrophysics Data System (ADS)

A long-term human flight needs a large-size space ships with artificial self-regulating ecological life-support system. The best way for creation of large-size space ship is a synthesis of light construction on Earth orbit, that does not need a high energy transportation carriers from Earth surface. The construction can be created by the way of chemical polymerisation reaction under space environment. But the space conditions are very specific for chemical reactions. A high vacuum, high energy particles, X-rays, UV- and VUV-irradiations, atomic oxygen, microgravity have a significant influence on chemical reactions. Polymerisation reactions in liquid active mixture were studied in simulated space environment. The epoxy resins based on Bisphenol A and amine curing agents were investigated under vacuum, microwave plasma discharge and ion beam. An acceleration of polymerisation reaction with free radicals formation was observed. The polymerisation reaction can be carried out under space environment. The study was supported by Alexander von Humboldt Foundation (A. Kondyurin) and European Space Agency, ESTEC (contract 17083/03/NL/Sfe "Space Environmental Effects on the Polymerisation of Composite Structures").

Kondyurin, A.; Lauke, B.; Richter, E.

113

Detailed Chemical Kinetic Reaction Mechanisms for Incineration of Organophosphorus and Fluoro-Organophosphorus Compounds  

SciTech Connect

A detailed chemical kinetic reaction mechanism is developed to describe incineration of the chemical warfare nerve agent sarin (GB), based on commonly used principles of bond additivity and hierarchical reaction mechanisms. The mechanism is based on previous kinetic models of organophosphorus compounds such as TMP, DMMP and DIMP that are often used as surrogates to predict incineration of GB. Kinetic models of the three surrogates and GB are then used to predict their consumption in a perfectly stirred reactor fueled by natural gas to simulate incineration of these chemicals. Computed results indicate that DIMP is the only one of these surrogates that adequately describes combustion of GB under comparable conditions. The kinetic pathways responsible for these differences in reactivity are identified and discussed. The most important reaction in GB and DIMP that makes them more reactive than TMP or DMMP is found to be a six-center molecular elimination reaction producing propene.

Glaude, P A; Melius, C; Pitz, W J; Westbrook, C K

2001-12-13

114

40 CFR 799.19 - Chemical imports and exports.  

Code of Federal Regulations, 2013 CFR

...CONTINUED) IDENTIFICATION OF SPECIFIC CHEMICAL SUBSTANCE AND MIXTURE TESTING REQUIREMENTS General Provisions § 799.19 Chemical...Persons who export or who intend to export chemical substances or mixtures listed in subpart B, subpart C, or subpart D of this...

2013-07-01

115

Reduction of chemical reaction networks through delay distributions  

NASA Astrophysics Data System (ADS)

Accurate modelling and simulation of dynamic cellular events require two main ingredients: an adequate description of key chemical reactions and simulation of such chemical events in reasonable time spans. Quite logically, posing the right model is a crucial step for any endeavour in Computational Biology. However, more often than not, it is the associated computational costs which actually limit our capabilities of representing complex cellular behaviour. In this paper, we propose a methodology aimed at representing chains of chemical reactions by much simpler, reduced models. The abridgement is achieved by generation of model-specific delay distribution functions, consecutively fed to a delay stochastic simulation algorithm. We show how such delay distributions can be analytically described whenever the system is solely composed of consecutive first-order reactions, with or without additional ``backward'' bypass reactions, yielding an exact reduction. For models including other types of monomolecular reactions (constitutive synthesis, degradation, or ``forward'' bypass reactions), we discuss why one must adopt a numerical approach for its accurate stochastic representation, and propose two alternatives for this. In these cases, the accuracy depends on the respective numerical sample size. Our model reduction methodology yields significantly lower computational costs while retaining accuracy. Quite naturally, computational costs increase alongside network size and separation of time scales. Thus, we expect our model reduction methodologies to significantly decrease computational costs in these instances. We anticipate the use of delays in model reduction will greatly alleviate some of the current restrictions in simulating large sets of chemical reactions, largely applicable in pharmaceutical and biological research.

Barrio, Manuel; Leier, André; Marquez-Lago, Tatiana T.

2013-03-01

116

Radical-neutral chemical reactions studied at low temperature with VUV synchrotron photoionization mass spectrometry  

NASA Astrophysics Data System (ADS)

A pulsed Laval nozzle apparatus employing tunable VUV synchrotron photoionization and quadrupole mass spectrometry for the study of radical-neutral chemical reactions of importance for modeling the atmosphere of Titan and the outer planets is described. The apparatus enables the study of low-temperature kinetics and isomer-resolved product branching of highly reactive radicals with unsaturated hydrocarbons reactions. The low-temperature branching ratio for the reaction of the ethynyl radical (C2H) with allene (C3H4) has been measured for the first time at 79 K. This reaction is found to yield 1,4-pentadiyne as the major reaction product (50+10%), followed by ethynylallene (28+10%) and methyldiacetylene (22+10%) via H-atom elimination from the initially formed C5H5 adduct. The derived branching ratios can be directly used to predict the chemical evolution of Titan's atmosphere.

Soorkia, Satchin; Leone, Stephen R.; Wilson, Kevin R.

2012-11-01

117

Program Helps To Determine Chemical-Reaction Mechanisms  

NASA Technical Reports Server (NTRS)

General Chemical Kinetics and Sensitivity Analysis (LSENS) computer code developed for use in solving complex, homogeneous, gas-phase, chemical-kinetics problems. Provides for efficient and accurate chemical-kinetics computations and provides for sensitivity analysis for variety of problems, including problems involving honisothermal conditions. Incorporates mathematical models for static system, steady one-dimensional inviscid flow, reaction behind incident shock wave (with boundary-layer correction), and perfectly stirred reactor. Computations of equilibrium properties performed for following assigned states: enthalpy and pressure, temperature and pressure, internal energy and volume, and temperature and volume. Written in FORTRAN 77 with exception of NAMELIST extensions used for input.

Bittker, D. A.; Radhakrishnan, K.

1995-01-01

118

Importance of toxicokinetics for interspecies variation in sensitivity to chemicals.  

PubMed

Interspecies variation in sensitivity to synthetic chemicals can be orders of magnitude large. Species traits causing the variation can be related to toxicokinetics (uptake, distribution, biotransformation, elimination) or toxicodynamics (interaction with biological target sites). We present an approach to systematically measure and model the contribution of uptake, biotransformation, internal distribution, and elimination kinetics toward species sensitivity differences. The aim is to express sensitivity as target tissue specific, internal lethal concentrations. A case study with the pesticides diazinon, imidacloprid, and propiconazole and the aquatic invertebrates Gammarus pulex, Gammarus fossarum, and Lymnaea stagnalis illustrates the approach. L. stagnalis accumulates more pesticides than Gammaridae when measured in whole organisms but less in target tissues such as the nervous system. Toxicokinetics, i.e. biotransformation and distribution, explain the higher tolerance of L. stagnalis to the insecticide diazinon when compared to Gammaridae. L. stagnalis was again more tolerant to the other neurotoxicant imidacloprid; however, the difference in sensitivity could not be explained by toxicokinetics alone, indicating the importance of toxicodynamic differences. Sensitivity to propiconazole was comparable among all species and, when expressed as internal lethal concentrations, falls in the range of baseline toxicity. PMID:24758734

Nyman, Anna-Maija; Schirmer, Kristin; Ashauer, Roman

2014-05-20

119

Favorite Demonstration: Demonstrating Indigo Carmine Oxidation-Reduction Reactions--A Choreography for Chemical Reactions  

NSDL National Science Digital Library

The indigo carmine demonstration (Ferguson et al. 1973), also referred to as a traffic-light demonstration (Flinn Scientific 2007a), is an example of a set of oxidation-reduction reactions that occurs within one solution. This type of demonstration can be used to introduce the concept of chemical reaction to undergraduate nonscience majors. Through their observations guided by the instructor, students begin to develop and construct the following concepts: color changes, reaction rates, reversible reactions, energy requirements (endothermic/exothermic), and equilibrium.

Majerich, David M.; Schmuckler, Joseph S.

2008-03-01

120

2011 Chemical Reactions at Surfaces Gordon Research Conference  

SciTech Connect

The Gordon Research Conference on Chemical Reactions at Surfaces is dedicated to promoting and advancing the fundamental science of interfacial chemistry and physics by providing surface scientists with the foremost venue for presentation and discussion of research occurring at the frontiers of their fields.

Peter Stair

2011-02-11

121

Chemical Reactions at the Au/InP Interface.  

National Technical Information Service (NTIS)

Chemical reactions at the Au/InP interface were investigated in the temperature range 25 510C by x-ray diffractometry, scanning electron microscopy and energy dispersive X-ray analysis. The samples were prepared by depositing gold films onto clean InP(100...

C. T. Tsai R. S. Williams

1986-01-01

122

Pyrimidine-specific chemical reactions useful for DNA sequencing.  

PubMed Central

Potassium permanganate reacts selectively with thymidine residues in DNA (1) while hydroxylamine hydrochloride at pH 6 specifically attacks cytosine (2). We have adopted these reactions for use with the chemical sequencing method developed by Maxam and Gilbert (3). Images

Rubin, C M; Schmid, C W

1980-01-01

123

Chemical Reactions on Ice Surfaces; Experimental and Theoretical Studies  

NASA Astrophysics Data System (ADS)

Surfaces of ice particles in the stratosphere and troposphere are now widely recognized to heterogeneously catalyze chemical reactions that lead to changes in the chemical composition of the atmosphere, however, not much is known about the reaction mechanisms, their time scales, or why ice is required for the reactions. We address these issues with respect to heterogeneous chlorine activation reactions on ice which ultimately lead to stratospheric ozone depletion. Studies using second harmonic generation (SHG) are presented first. The equilibrium basal (0001) face of ice Ih is held under stratospherically relevant conditions, and chlorine activation reactions are studied using submonolayer reactant amounts. In another set of experiments, the adsorption of HCl on ice, a key step required for chlorine activation, is examined using chemical ionization mass spectrometry (CIMS). The ice is formed by freezing from the melt inside a flow tube reactor and is polycrystalline; HCl partial pressures are held between 1 10^9 and 1 x 10^6 Torr, and the experiments are performed under stratospherically relevant temperature and pressure conditions. Finally, ab initio calculations on water clusters representative of an adsorption site on the basal ice surface and Car-Parrinello Molecular Dynamics (CPMD) simulations on ice slabs having a varying number of dangling OH groups help us develop a molecular level picture of how ice surfaces are involved in stratospheric ozone depletion.

Geiger, Franz

2002-03-01

124

Mixing, chemical reaction and flow field development in ducted rockets  

SciTech Connect

Calculations have been made of the three-dimensional mixing, chemical reaction, and flow field development in a typical ducted rocket configuration. The governing partial differential equations are numerically solved by an iterative finite-difference solution procedure. The physical models include the k approx. epsilon turbulence model, one-step reaction, and mixing controlled chemical reaction rate. Radiation is neglected. The mean flow structure, fuel dispersal patterns, and temperature field are presented in detail for a base configuration with 0.058 m (2 in.) dome height, 45/sup 0/ side arm inclination, and with gaseous ethylene injected from the dome plate at an eccentric location. In addition, the influences of the geometrical parameters such as dome height, inclination of the side arms, and location of the fuel injector are studied.

Vanka, S.P.; Craig, R.R.; Stull, F.D.

1984-09-01

125

Chemical reaction mediated self-assembly of PTCDA into nanofibers  

NASA Astrophysics Data System (ADS)

Uniform and crystalline nanofibers of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), an insoluble organic semiconducting molecule, have been achieved by self-assembling the molecules using chemical reaction mediated conversion of an appropriately designed soluble precursor, perylene tetracarboxylic acid (PTCA) using carbodiimide chemistry.Uniform and crystalline nanofibers of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), an insoluble organic semiconducting molecule, have been achieved by self-assembling the molecules using chemical reaction mediated conversion of an appropriately designed soluble precursor, perylene tetracarboxylic acid (PTCA) using carbodiimide chemistry. Electronic supplementary information (ESI) available: Synthesis protocols and schemes, plausible mechanism of reaction, additional TEM images, TEM images for stability of fibers after extraction in DCM and aqueous potassium carbonate, detailed FTIR spectra, mass spectroscopy, and XRD of fibers of PTCDA. See DOI: 10.1039/c1nr10579e

Sayyad, Arshad S.; Balakrishnan, Kaushik; Ajayan, Pulickel M.

2011-09-01

126

Robust Stochastic Chemical Reaction Networks and Bounded Tau-Leaping  

PubMed Central

Abstract The behavior of some stochastic chemical reaction networks is largely unaffected by slight inaccuracies in reaction rates. We formalize the robustness of state probabilities to reaction rate deviations, and describe a formal connection between robustness and efficiency of simulation. Without robustness guarantees, stochastic simulation seems to require computational time proportional to the total number of reaction events. Even if the concentration (molecular count per volume) stays bounded, the number of reaction events can be linear in the duration of simulated time and total molecular count. We show that the behavior of robust systems can be predicted such that the computational work scales linearly with the duration of simulated time and concentration, and only polylogarithmically in the total molecular count. Thus our asymptotic analysis captures the dramatic speedup when molecular counts are large, and shows that for bounded concentrations the computation time is essentially invariant with molecular count. Finally, by noticing that even robust stochastic chemical reaction networks are capable of embedding complex computational problems, we argue that the linear dependence on simulated time and concentration is likely optimal.

2009-01-01

127

THE IMPORTANCE OF SPATIAL ACCURACY FOR CHEMICAL INFORMATION MANAGEMENT  

EPA Science Inventory

Information about chemicals can be critical to making timely decisions. The results of these decisions may not be realized for many years. In order to increase the value of chemical information and to create and utilize meaningful environmental models, the Environmental Prote...

128

Information-Theoretical Complexity Analysis of Selected Elementary Chemical Reactions  

NASA Astrophysics Data System (ADS)

We investigate the complexity of selected elementary chemical reactions (namely, the hydrogenic-abstraction reaction and the identity SN2 exchange reaction) by means of the following single and composite information-theoretic measures: disequilibrium (D), exponential entropy(L), Fisher information (I), power entropy (J), I-D, D-L and I-J planes and Fisher-Shannon (FS) and Lopez-Mancini-Calbet (LMC) shape complexities. These quantities, which are functionals of the one-particle density, are computed in both position (r) and momentum (p) spaces. The analysis revealed that the chemically significant regions of these reactions can be identified through most of the single information-theoretic measures and the two-component planes, not only the ones which are commonly revealed by the energy, such as the reactant/product (R/P) and the transition state (TS), but also those that are not present in the energy profile such as the bond cleavage energy region (BCER), the bond breaking/forming regions (B-B/F) and the charge transfer process (CT). The analysis of the complexities shows that the energy profile of the abstraction reaction bears the same information-theoretical features of the LMC and FS measures, however for the identity SN2 exchange reaction does not hold a simple behavior with respect to the LMC and FS measures. Most of the chemical features of interest (BCER, B-B/F and CT) are only revealed when particular information-theoretic aspects of localizability (L or J), uniformity (D) and disorder (I) are considered.

Molina-Espíritu, M.; Esquivel, R. O.; Dehesa, J. S.

129

Investigation of chemical reactions in solution using API-MS  

NASA Astrophysics Data System (ADS)

The general concepts, advantages, and applications of on-line and off-line screening to organic reaction mechanistic studies applying API-MS are reviewed. An overview is presented of the development and the present stage of connected microreactors to API ion-sources. Examples of the successful application of API in revealing, elucidating, and helping to consolidate several proposed mechanisms of organic reactions are summarized. Finally, a variety of outstanding features and advantages that make API-MS the most suitable tool for the fast screening of intermediates directly from solution, and the exceptional gains in chemical information for organic chemists are also emphasized.

Santos, Leonardo Silva; Knaack, Larissa; Metzger, Jürgen O.

2005-11-01

130

Electrochemical Reactions During Ru Chemical Mechanical Planarization and Safety Considerations  

NASA Astrophysics Data System (ADS)

We analyzed electrochemical reactions during ruthenium (Ru) chemical mechanical planarization (CMP) using a potentiostat and a quartz crystal microbalance, and considered the potential safety issues. We evaluated the valence number derived from Faraday's law using the dissolution mass change of Ru and total coulomb consumption in the electrochemical reactions for Ru in acidic solution and slurry. The valence numbers of dissolved Ru ions were distributed in the range of 2 to 3.5. As toxic ruthenium tetroxide (RuO4) has a valence number of 8, we were able to conclude that no toxic RuO4 was produced in the actual Ru CMP.

Shima, Shohei; Wada, Yutaka; Tokushige, Katsuhiko; Fukunaga, Akira; Tsujimura, Manabu

2011-05-01

131

Chemical reactions studied at ultra-low temperature in liquid helium clusters  

SciTech Connect

Low-temperature reaction rates are important ingredients for astrophysical reaction networks modeling the formation of interstellar matter in molecular clouds. Unfortunately, such data is difficult to obtain by experimental means. In an attempt to study low-temperature reactions of astrophysical interest, we have investigated relevant reactions at ultralow temperature in liquid helium droplets. Being prepared by supersonic expansion of helium gas at high pressure through a nozzle into a vacuum, large helium clusters in the form of liquid droplets constitute nano-sized reaction vessels for the study of chemical reactions at ultra-low temperature. If the normal isotope {sup 4}He is used, the helium droplets are superfluid and characterized by a constant temperature of 0.37 K. Here we present results obtained for Mg, Al, and Si reacting with O{sub 2}. Mass spectrometry was employed to characterize the reaction products. As it may be difficult to distinguish between reactions occurring in the helium droplets before they are ionized and ion-molecule reactions taking place after the ionization, additional techniques were applied to ensure that the reactions actually occurred in the helium droplets. This information was provided by measuring the chemiluminescence light emitted by the products, the evaporation of helium atoms by the release of the reaction heat, or by laser-spectroscopic identification of the reactants and products.

Huisken, Friedrich; Krasnokutski, Serge A. [Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the University of Jena, Institute of Solid State Physics, Helmholtzweg 3, D-07743 Jena (Germany)

2012-11-27

132

Chemical reactions studied at ultra-low temperature in liquid helium clusters  

NASA Astrophysics Data System (ADS)

Low-temperature reaction rates are important ingredients for astrophysical reaction networks modeling the formation of interstellar matter in molecular clouds. Unfortunately, such data is difficult to obtain by experimental means. In an attempt to study low-temperature reactions of astrophysical interest, we have investigated relevant reactions at ultralow temperature in liquid helium droplets. Being prepared by supersonic expansion of helium gas at high pressure through a nozzle into a vacuum, large helium clusters in the form of liquid droplets constitute nano-sized reaction vessels for the study of chemical reactions at ultra-low temperature. If the normal isotope 4He is used, the helium droplets are superfluid and characterized by a constant temperature of 0.37 K. Here we present results obtained for Mg, Al, and Si reacting with O2. Mass spectrometry was employed to characterize the reaction products. As it may be difficult to distinguish between reactions occurring in the helium droplets before they are ionized and ion-molecule reactions taking place after the ionization, additional techniques were applied to ensure that the reactions actually occurred in the helium droplets. This information was provided by measuring the chemiluminescence light emitted by the products, the evaporation of helium atoms by the release of the reaction heat, or by laser-spectroscopic identification of the reactants and products.

Huisken, Friedrich; Krasnokutski, Serge A.

2012-11-01

133

A microvascular system for chemical reactions using surface waste heat.  

PubMed

Coffee-powered chemistry: Low-grade waste heat on surfaces can be used to drive chemical reactions, including the regeneration of a CO2 capture solution. Flowing two-phase heat transfer has been implemented within microvascular systems. This stripping system can be adapted to pre-fabricated surfaces, as demonstrated by a coffee mug containing a 1.2?m long microchannel. MEA=monoethanolamine. PMID:24307247

Nguyen, Du Thai; Esser-Kahn, Aaron P

2013-12-16

134

Separation of the isotopes of boron by chemical exchange reactions  

DOEpatents

The isotopes of boron, .sup.10 B and .sup.11 B, are separated by means of a gas-liquid chemical exchange reaction involving the isotopic equilibrium between gaseous BF.sub.3 and a liquid BF.sub.3 . donor molecular addition complex formed between BF.sub.3 gas and a donor chosen from the group consisting of: nitromethane, acetone, methyl isobutyl ketone, or diisobutyl ketone.

McCandless, Frank P. (Bozeman, MT); Herbst, Ronald S. (Idaho Falls, ID)

1995-01-01

135

Stochastic Generator of Chemical Structure. 3. Reaction Network Generation  

SciTech Connect

A new method to generate chemical reaction network is proposed. The particularity of the method is that network generation and mechanism reduction are performed simultaneously using sampling techniques. Our method is tested for hydrocarbon thermal cracking. Results and theoretical arguments demonstrate that our method scales in polynomial time while other deterministic network generator scale in exponential time. This finding offers the possibility to investigate complex reacting systems such as those studied in petroleum refining and combustion.

FAULON,JEAN-LOUP; SAULT,ALLEN G.

2000-07-15

136

Exploring chemical diversity via a modular reaction pairing strategy  

PubMed Central

Summary The efficient synthesis of an 80-member library of unique benzoxathiazocine 1,1-dioxides by a microwave-assisted, intermolecular nucleophilic aromatic substitution (SNAr) diversification pathway is reported. Eight benzofused sultam cores were generated by means of a sulfonylation/SNAr/Mitsunobu reaction pairing protocol, and subsequently diversified by intermolecular SNAr with ten chiral, non-racemic amine/amino alcohol building blocks. Computational analyses were employed to explore and evaluate the chemical diversity of the library.

Loh, Joanna K; Yoon, Sun Young; Samarakoon, Thiwanka B; Rolfe, Alan; Porubsky, Patrick; Neuenswander, Benjamin; Lushington, Gerald H

2012-01-01

137

Exploring chemical reaction mechanisms through harmonic Fourier beads path optimization  

NASA Astrophysics Data System (ADS)

Here, we apply the harmonic Fourier beads (HFB) path optimization method to study chemical reactions involving covalent bond breaking and forming on quantum mechanical (QM) and hybrid QM/molecular mechanical (QM/MM) potential energy surfaces. To improve efficiency of the path optimization on such computationally demanding potentials, we combined HFB with conjugate gradient (CG) optimization. The combined CG-HFB method was used to study two biologically relevant reactions, namely, L- to D-alanine amino acid inversion and alcohol acylation by amides. The optimized paths revealed several unexpected reaction steps in the gas phase. For example, on the B3LYP/6-31G(d,p) potential, we found that alanine inversion proceeded via previously unknown intermediates, 2-iminopropane-1,1-diol and 3-amino-3-methyloxiran-2-ol. The CG-HFB method accurately located transition states, aiding in the interpretation of complex reaction mechanisms. Thus, on the B3LYP/6-31G(d,p) potential, the gas phase activation barriers for the inversion and acylation reactions were 50.5 and 39.9 kcal/mol, respectively. These barriers determine the spontaneous loss of amino acid chirality and cleavage of peptide bonds in proteins. We conclude that the combined CG-HFB method further advances QM and QM/MM studies of reaction mechanisms.

Khavrutskii, Ilja V.; Smith, Jason B.; Wallqvist, Anders

2013-10-01

138

Chemical Characterization and Reactivity of Fuel-Oxidizer Reaction Product  

NASA Technical Reports Server (NTRS)

Fuel-oxidizer reaction product (FORP), the product of incomplete reaction of monomethylhydrazine and nitrogen tetroxide propellants prepared under laboratory conditions and from firings of Shuttle Reaction Control System thrusters, has been characterized by chemical and thermal analysis. The composition of FORP is variable but falls within a limited range of compositions that depend on three factors: the fuel-oxidizer ratio at the time of formation; whether the composition of the post-formation atmosphere is reducing or oxidizing; and the reaction or post-reaction temperature. A typical composition contains methylhydrazinium nitrate, ammonium nitrate, methylammonium nitrate, and trace amounts of hydrazinium nitrate and 1,1-dimethylhydrazinium nitrate. Thermal decomposition reactions of the FORP compositions used in this study were unremarkable. Neither the various compositions of FORP, the pure major components of FORP, nor mixtures of FORP with propellant system corrosion products showed any unusual thermal activity when decomposed under laboratory conditions. Off-limit thruster operations were simulated by rapid mixing of liquid monomethylhydrazine and liquid nitrogen tetroxide in a confined space. These tests demonstrated that monomethylhydrazine, methylhydrazinium nitrate, ammonium nitrate, or Inconel corrosion products can induce a mixture of monomethylhydrazine and nitrogen tetroxide to produce component-damaging energies. Damaging events required FORP or metal salts to be present at the initial mixing of monomethylhydrazine and nitrogen tetroxide.

David, Dennis D.; Dee, Louis A.; Beeson, Harold D.

1997-01-01

139

Automation of the Maxam-Gilbert chemical sequencing reactions.  

PubMed

A practical automated method of Maxam-Gilbert chemical sequencing reactions that uses solid-phase chromatography methods to purify DNA following chemical modification and cleavage is described in this report. The automation has primarily been made possible by using specially designed BioPak mini-columns, compatible with the Biomek 1000 automated workstation, which can be utilized in a manner similar to that of standard pipet tips. This automated chromatographic sequencing method produces rapid and reliable data as verified by sequencing a known human factor IX exon VIII gene fragment. The procedure presented in this report is a prototype for a single-fragment reaction and can easily be expanded to perform reactions on as many as 8 fragments at a time. The automation eliminates the tedious and time-consuming steps in the original method and increases the rate of sequence acquisition. This technology makes the Maxam-Gilbert chemical sequencing protocol more accessible, especially in large-scale, automated sequencing projects. PMID:8074875

Boland, E J; Pillai, A; Odom, M W; Jagadeeswaran, P

1994-06-01

140

APOLLO: A computer program for the calculation of chemical equilibrium and reaction kinetics of chemical systems  

SciTech Connect

Several of the technologies being evaluated for the treatment of waste material involve chemical reactions. Our example is the in situ vitrification (ISV) process where electrical energy is used to melt soil and waste into a glass like'' material that immobilizes and encapsulates any residual waste. During the ISV process, various chemical reactions may occur that produce significant amounts of products which must be contained and treated. The APOLLO program was developed to assist in predicting the composition of the gases that are formed. Although the development of this program was directed toward ISV applications, it should be applicable to other technologies where chemical reactions are of interest. This document presents the mathematical methodology of the APOLLO computer code. APOLLO is a computer code that calculates the products of both equilibrium and kinetic chemical reactions. The current version, written in FORTRAN, is readily adaptable to existing transport programs designed for the analysis of chemically reacting flow systems. Separate subroutines EQREACT and KIREACT for equilibrium ad kinetic chemistry respectively have been developed. A full detailed description of the numerical techniques used, which include both Lagrange multiplies and a third-order integrating scheme is presented. Sample test problems are presented and the results are in excellent agreement with those reported in the literature.

Nguyen, H.D.

1991-11-01

141

APOLLO: A computer program for the calculation of chemical equilibrium and reaction kinetics of chemical systems  

SciTech Connect

Several of the technologies being evaluated for the treatment of waste material involve chemical reactions. Our example is the in situ vitrification (ISV) process where electrical energy is used to melt soil and waste into a ``glass like`` material that immobilizes and encapsulates any residual waste. During the ISV process, various chemical reactions may occur that produce significant amounts of products which must be contained and treated. The APOLLO program was developed to assist in predicting the composition of the gases that are formed. Although the development of this program was directed toward ISV applications, it should be applicable to other technologies where chemical reactions are of interest. This document presents the mathematical methodology of the APOLLO computer code. APOLLO is a computer code that calculates the products of both equilibrium and kinetic chemical reactions. The current version, written in FORTRAN, is readily adaptable to existing transport programs designed for the analysis of chemically reacting flow systems. Separate subroutines EQREACT and KIREACT for equilibrium ad kinetic chemistry respectively have been developed. A full detailed description of the numerical techniques used, which include both Lagrange multiplies and a third-order integrating scheme is presented. Sample test problems are presented and the results are in excellent agreement with those reported in the literature.

Nguyen, H.D.

1991-11-01

142

Physio-chemical reactions in recycle aggregate concrete.  

PubMed

Concrete waste constitutes the major proportion of construction waste at about 50% of the total waste generated. An effective way to reduce concrete waste is to reuse it as recycled aggregate (RA) for the production of recycled aggregate concrete (RAC). This paper studies the physio-chemical reactions of cement paste around aggregate for normal aggregate concrete (NAC) and RAC mixed with normal mixing approach (NMA) and two-stage mixing approach (TSMA) by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Four kinds of physio-chemical reactions have been recorded from the concrete samples, including the dehydration of C(3)S(2)H(3), iron-substituted ettringite, dehydroxylation of CH and development of C(6)S(3)H at about 90 degrees C, 135 degrees C, 441 degrees C and 570 degrees C, respectively. From the DSC results, it is confirmed that the concrete samples with RA substitution have generated less amount of strength enhancement chemical products when compared to those without RA substitution. However, the results from the TSMA are found improving the RAC quality. The pre-mix procedure of the TSMA can effectively develop some strength enhancing chemical products including, C(3)S(2)H(3), ettringite, CH and C(6)S(3)H, which shows that RAC made from the TSMA can improve the hydration processes. PMID:18718710

Tam, Vivian W Y; Gao, X F; Tam, C M; Ng, K M

2009-04-30

143

Reversible chemical reactions for single-color multiplexing microscopy.  

PubMed

Recent developments in biology demand an increasing number of simultaneously imaged structures with standard fluorescence microscopy. However, the number of multiplexed channels is limited for most multiplexing modalities, such as spectral multiplexing or fluorescence-lifetime imaging. We propose extending the number of imaging channels by using chemical reactions, controlling the emissive state of fluorescent dyes. As proof of concept, we reversibly switch a fluorescent copper sensor to enable successive imaging of two different structures in the same spectral channel. We also show that this chemical multiplexing is orthogonal to existing methods. By using two different dyes, we combine chemical with spectral multiplexing for the simultaneous imaging of four different structures with only two spectrally different channels. We characterize and discuss the approach and provide perspectives for extending imaging modalities in stimulated emission depletion microscopy, for which spectral multiplexing is technically demanding. PMID:24753024

Brox, Dominik; Schwering, Michael; Engelhardt, Johann; Herten, Dirk-Peter

2014-08-01

144

Computational Analyses of Complex Flows with Chemical Reactions  

NASA Astrophysics Data System (ADS)

The heat and mass transfer phenomena in micro-scale for the mass transfer phenomena on drug in cylindrical matrix system, the simulation of oxygen/drug diffusion in a three dimensional capillary network, and a reduced chemical kinetic modeling of gas turbine combustion for Jet propellant-10 have been studied numerically. For the numerical analysis of the mass transfer phenomena on drug in cylindrical matrix system, the governing equations are derived from the cylindrical matrix systems, Krogh cylinder model, which modeling system is comprised of a capillary to a surrounding cylinder tissue along with the arterial distance to veins. ADI (Alternative Direction Implicit) scheme and Thomas algorithm are applied to solve the nonlinear partial differential equations (PDEs). This study shows that the important factors which have an effect on the drug penetration depth to the tissue are the mass diffusivity and the consumption of relevant species during the time allowed for diffusion to the brain tissue. Also, a computational fluid dynamics (CFD) model has been developed to simulate the blood flow and oxygen/drug diffusion in a three dimensional capillary network, which are satisfied in the physiological range of a typical capillary. A three dimensional geometry has been constructed to replicate the one studied by Secomb et al. (2000), and the computational framework features a non-Newtonian viscosity model for blood, the oxygen transport model including in oxygen-hemoglobin dissociation and wall flux due to tissue absorption, as well as an ability to study the diffusion of drugs and other materials in the capillary streams. Finally, a chemical kinetic mechanism of JP-10 has been compiled and validated for a wide range of combustion regimes, covering pressures of 1atm to 40atm with temperature ranges of 1,200 K--1,700 K, which is being studied as a possible Jet propellant for the Pulse Detonation Engine (PDE) and other high-speed flight applications such as hypersonic missiles. The comprehensive skeletal mechanism consists of 58 species and 315 reactions including in CPD, Benzene formation process by the theory for polycyclic aromatic hydrocarbons (PAH) and soot formation process on the constant volume combustor, premixed flame characteristics.

Bae, Kang-Sik

145

Chemical fibres — An important factor in economic development  

Microsoft Academic Search

1997 set a record for the past 20 years in the growth rate of world production of chemical fibres and filaments. In comparison\\u000a to 1996, this figure, was 11%, primarily due to synthetic fibres, where polyester fibres still occupy the leading position\\u000a (approximately 15 million tons). Production of cellulose (viscose, acetate, cuprammonium) fibres and filaments dropped, especially\\u000a for industrial applications.

E. M. Aizenshtein

1998-01-01

146

Significance of vapor phase chemical reactions on CVD rates predicted by chemically frozen and local thermochemical equilibrium boundary layer theories  

NASA Technical Reports Server (NTRS)

This paper investigates the role played by vapor-phase chemical reactions on CVD rates by comparing the results of two extreme theories developed to predict CVD mass transport rates in the absence of interfacial kinetic barrier: one based on chemically frozen boundary layer and the other based on local thermochemical equilibrium. Both theories consider laminar convective-diffusion boundary layers at high Reynolds numbers and include thermal (Soret) diffusion and variable property effects. As an example, Na2SO4 deposition was studied. It was found that gas phase reactions have no important role on Na2SO4 deposition rates and on the predictions of the theories. The implications of the predictions of the two theories to other CVD systems are discussed.

Gokoglu, Suleyman A.

1988-01-01

147

The influence of overlap interactions on chemical reactions in confinement  

NASA Astrophysics Data System (ADS)

Chemical reactions are often carried out in nano-structured materials due to their large surface area per unit mass. It is, however, difficult to understand fully the role of the nano-structure in many reactions due to the superposition of multiple effects. Such effects include: the reduced dimensionality of the system, the heterogeneity of the pore surfaces, the selective adsorption of reactants/products, catalytic effects, and transport limitations. Experimental studies often show many of these effects at the same time, making the results difficult to interpret. In this work we present results of density functional theory calculations illustrating the influence of overlap interactions (shape-catalytic effects) on chemical reactions. In particular, we show the effect of confinement in small pores on the rates of rotational isomerizations of n-butane, 1-butene and 1,3-butadiene. We find that the rates of these transitions change as the double exponential of the pore size in the molecular sieving limit. These results are a first step towards an integrated model for the design of catalytic materials.

Santiso, Erik E.; Gubbins, Keith E.; George, Aaron M.; Buongiorno Nardelli, Marco

2007-03-01

148

Nuclear field shift effect in chemical exchange reactions  

NASA Astrophysics Data System (ADS)

The classic theory of stable isotope fractionation in chemical exchange reactions has been established by Bigeleisen, Mayer, and Urey in 1947. The theory was based on the difference of molecular vibrational energies of isotopomers that are proportional to the respective masses, and hence, results in mass-dependent isotope effect only. In 1996, this conventional mass-dependent theory has been expanded by Bigeleisen to include a mass-independent term named the nuclear field shift effect. The nuclear field shift is an isotope shift in orbital electrons, which results from the isotopic difference in nuclear size and shape. The new equation defined by Bigeleisen (at a constant temperature) can be simply expressed as, ln ? = ? A + (? m/mm') B, where ? is the isotope separation factor, ? isotopic difference in mean-square nuclear charge radius, ?m difference between isotopic masses m and m'. A and B are scaling factors of the nuclear field shift effect and the conventional mass effect, respectively. Since this new theory was presented, the mass-independent isotope fractionation of various elements, e.g, Ti, Cr, Ni, Zn, Sr, Zr, Mo, Ru, Cd, Te, Ba, Nd, Sm, Gd, Yb, and U, found in chemical exchange systems has been successfully explained as the nuclear field shift effect. In our most recent studies, the nuclear field shift effect of Cr, Mo, Ru, Cd, and Te isotopes has been found in laboratory scale experiments. The isotopes of these elements were fractionated by using a liquid-liquid extraction system (a ligand exchange system) at room temperature. The isotopic analysis was performed by the multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) with a typical precision of <100 ppm (at ENS Lyon or UC Davis). Isotope enrichment factors did not show mass-dependent trend, but possessed a similar variation of their nuclear charge radii. For Cr, we tested a different chemical exchange system (a redox system): at high temperature (723-1023 K), an eutectic melt was contacted with a liquid metal. In this system, the nuclear field shift effect of Cr was also found. All these experimental results suggest that the nuclear field shift effect may occur in every chemical exchange reaction at various temperatures to various degrees. Therefore, isotopic anomalies found in a natural system might be partly or largely affected by the nuclear field shift effect via chemical reactions occurred in the nature. In order to clarify the degree and significance of its contribution, we may need to pay more attention to the nuclear field shift effect created chemically.

Fujii, T.; Moynier, F.; Yin, Q.; Albarède, F.

2007-12-01

149

Chemical characteristics of mineral trioxide aggregate and its hydration reaction  

PubMed Central

Mineral trioxide aggregate (MTA) was developed in early 1990s and has been successfully used for root perforation repair, root end filling, and one-visit apexification. MTA is composed mainly of tricalcium silicate and dicalcium silicate. When MTA is hydrated, calcium silicate hydrate (CSH) and calcium hydroxide is formed. Formed calcium hydroxide interacts with the phosphate ion in body fluid and form amorphous calcium phosphate (ACP) which finally transforms into calcium deficient hydroxyapatite (CDHA). These mineral precipitate were reported to form the MTA-dentin interfacial layer which enhances the sealing ability of MTA. Clinically, the use of zinc oxide euginol (ZOE) based materials may retard the setting of MTA. Also, the use of acids or contact with excessive blood should be avoided before complete set of MTA, because these conditions could adversely affect the hydration reaction of MTA. Further studies on the chemical nature of MTA hydration reaction are needed.

2012-01-01

150

On the importance of ions and ion-molecule reactions to plasma-surface interface reactions.  

PubMed

Ions are known to be key players in many plasma processes, including anisotropic etching, film deposition and surface modification. The relationship between plasma ions, film properties, and surface interactions of other plasma species is not, however, well known. Using our Imaging of Radicals Interacting with Surfaces (IRIS) technique, along with plasma ion mass spectrometry (PI-MS), and surface analysis data, we have measured the effects of ion bombardment on the surface interactions of SiF2 in SiF4 plasmas and of CF2 in C3F8 and C4F8 plasmas. SiF2 is a known product of F-atom etching of Si, and CF2 has also been cited as a product of fluorocarbon etching of Si. With both molecules, we measure surface generation when the surface is bombarded by all the plasma species. Removal of ions from the plasma molecular beam results in a net decrease in surface generation for both molecules at all powers. Results in both systems are compared with the gas-phase ion-molecule reaction data of Armentrout and coworkers. Preliminary guided-ion beam mass spectrometry results taken in the Armentrout laboratories for the Ar+ + C3F8 reaction system are also presented. PMID:12019976

Williams, Keri L; Martin, Ina T; Fisher, Ellen R

2002-05-01

151

Anthropogenic reaction parameters - the missing link between chemical intuition and the available chemical space.  

PubMed

How do skilled synthetic chemists develop good intuitive expertise? Why can we only access such a small amount of the available chemical space-both in terms of the reactions used and the chemical scaffolds we make? We argue here that these seemingly unrelated questions have a common root and are strongly interdependent. We performed a comprehensive analysis of organic reaction parameters dating back to 1771 and discovered that there are several anthropogenic factors that limit reaction parameters and thus the scope of synthetic chemistry. Nevertheless, many of the anthropogenic limitations such as narrow parameter space and the opportunity for rapid and clear feedback on the progress of reactions appear to be crucial for the acquisition of valid and reliable chemical intuition. In parallel, however, all of these same factors represent limitations for the exploration of available chemistry space and we argue that these are thus at least partly responsible for limited access to new chemistries. We advocate, therefore, that the present anthropogenic boundaries can be expanded by a more conscious exploration of "off-road" chemistry that would also extend the intuitive knowledge of trained chemists. PMID:24877159

Keser?, György M; Soós, Tibor; Kappe, C Oliver

2014-07-01

152

Computed Potential Energy Surfaces and Minimum Energy Pathway for Chemical Reactions  

NASA Technical Reports Server (NTRS)

Computed potential energy surfaces are often required for computation of such observables as rate constants as a function of temperature, product branching ratios, and other detailed properties. We have found that computation of the stationary points/reaction pathways using CASSCF/derivative methods, followed by use of the internally contracted CI method with the Dunning correlation consistent basis sets to obtain accurate energetics, gives useful results for a number of chemically important systems. Applications to complex reactions leading to NO and soot formation in hydrocarbon combustion are discussed.

Walch, Stephen P.; Langhoff, S. R. (Technical Monitor)

1994-01-01

153

Kinetics and thermodynamics of chemical reactions in Li/SOCl2 cells  

NASA Technical Reports Server (NTRS)

Work is described that was designed to determine the kinetic constants necessary to extrapolate kinetic data on Li/SOCl2 cells over the temperature range from 25 to 75 C. A second objective was to characterize as far as possible the chemical reactions that occur in the cells since these reactions may be important in understanding the potential hazards of these cells. The kinetics of the corrosion processes in undischarged Li/SOCl2 cells were determined and separated according to their occurrence at the anode and cathode; the effects that switching the current on and off has on the corrosion reactions was determined; and the effects of discharge state on the kinetics of the corrosion process were found. A thermodynamic analysis of the current-producing reactions in the cell was done and is included.

Hansen, Lee D.; Frank, Harvey

1987-01-01

154

Laser enhanced chemical reaction studies. Technical progress report  

SciTech Connect

A powerful infrared diode laser probe was used to determine final states of small (2-5 atom) molecules produced by collisions, photofragmentation, or chemical reactions with spectral resolution 0.0003 cm{sup {minus}1} and time resolution 10{sup {minus}7}s. Besides picturing the vibrational rotational quantum states of product molecules, this also provides a picture of the translational motion of recoiling fragments through the infrared Doppler line width profile. This method was used to probe collisions between cool bath molecules and vibrationally hot molecules, in order to understand the quenching mechanism for unimolecular chemical reactions. Long-range collisions appear to dominate production of bath molecules which become vibrationally excited during this quenching process. Glimpses are being provided of the separate behavior of translational and rotational degrees of freedom of recoiling bath molecules during relaxation of highly vibrationally excited donors. A study was completed of collisions between hot H atoms and CO{sub 2}, by measuring probability for excitation of the antisymmetric vibrational overtone level CO{sub 2}(00{sup 0}2). Comparison with a 00{sup 0}1 fundamental level study suggests that translational and rotational energy distributions in this collision can be described by classical mechanics, but that vibrational excitation probabilities require full quantum treatment. Relaxation of hot pyrazine by CO{sub 2} was studied. Multiphoton ionization studies have been begun.

Flynn, G.

1993-12-31

155

Detailed Chemical Kinetic Reaction Mechanisms for Combustion of Isomers of Heptane  

SciTech Connect

Detailed chemical kinetic reaction mechanisms are developed for all nine chemical isomers of heptane (C{sub 7}H{sub 16}), following techniques and models developed previously for other smaller alkane hydrocarbon species. These reaction mechanisms are tested at high temperatures by computing shock tube ignition delay times and at lower temperatures by simulating ignition in a rapid compression machine. Although the corresponding experiments have not been reported in the literature for most of these isomers of heptane, intercomparisons between the computed results for these isomers and comparisons with available experimental results for other alkane fuels are used to validate the reaction mechanisms as much as possible. Differences in the overall reaction rates of these fuels are discussed in terms of differences in their molecular structure and the resulting variations in rates of important elementary reactions. Reaction mechanisms in this study are works in progress and the results reported here are subject to change, based on model improvements and corrections of errors not yet discovered.

Westbrook, C K; Pitz, W J; Curran, H C; Boercker, J; Kunrath, E

2001-03-26

156

Following a chemical reaction using high-harmonic interferometry.  

PubMed

The study of chemical reactions on the molecular (femtosecond) timescale typically uses pump laser pulses to excite molecules and subsequent probe pulses to interrogate them. The ultrashort pump pulse can excite only a small fraction of molecules, and the probe wavelength must be carefully chosen to discriminate between excited and unexcited molecules. The past decade has seen the emergence of new methods that are also aimed at imaging chemical reactions as they occur, based on X-ray diffraction, electron diffraction or laser-induced recollision--with spectral selection not available for any of these new methods. Here we show that in the case of high-harmonic spectroscopy based on recollision, this apparent limitation becomes a major advantage owing to the coherent nature of the attosecond high-harmonic pulse generation. The coherence allows the unexcited molecules to act as local oscillators against which the dynamics are observed, so a transient grating technique can be used to reconstruct the amplitude and phase of emission from the excited molecules. We then extract structural information from the amplitude, which encodes the internuclear separation, by quantum interference at short times and by scattering of the recollision electron at longer times. The phase records the attosecond dynamics of the electrons, giving access to the evolving ionization potentials and the electronic structure of the transient molecule. In our experiment, we are able to document a temporal shift of the high-harmonic field of less than an attosecond (1 as = 10(-18) s) between the stretched and compressed geometry of weakly vibrationally excited Br(2) in the electronic ground state. The ability to probe structural and electronic features, combined with high time resolution, make high-harmonic spectroscopy ideally suited to measuring coupled electronic and nuclear dynamics occurring in photochemical reactions and to characterizing the electronic structure of transition states. PMID:20671706

Wörner, H J; Bertrand, J B; Kartashov, D V; Corkum, P B; Villeneuve, D M

2010-07-29

157

Irreversible bimolecular chemical reactions on directed scale-free networks.  

PubMed

Kinetics of irreversible bimolecular chemical reactions A+A?0 and A+B?0 on directed scale-free networks with the in-degree distribution P(in)(k)?k(-?)(in) and the out-degree distribution P(out)(?)??(-?)(out) are investigated. Since the correlation between k and ? of each node generally exists in directed networks, we control the correlation (k?) with the probability r?[0,1] by two different algorithms for the construction of the directed networks, i.e., the so-called k and ? algorithms. For r=1, the k algorithm gives (k?)=(k(2)), whereas the ? algorithm gives (k?)=(?(2). For r=0, (k?)=(k)(?) for both algorithms. The kinetics of both reactions are analyzed using heterogeneous mean-field (HMF) theory and Monte Carlo simulations. The density of particles (?) algebraically decays in time t as ?(t)?t(-?). The kinetics of both reactions are determined by the same rate equation, d?/dt=a?(2)+b?(?-1), apart from coefficients. The exponent ? is determined by the algorithm: ?=?(in) for the k algorithm (r?0) and ?=?(min) for the ? algorithm (r>0), where ?(min) is the smaller exponent between ?(in) and ?(out). For ?>3, one observes the ordinary mean-field kinetics, ??1/t (?=1). In contrast, for ?<3, ?(t) anomalously decays with ?=1/(?-2). The HMF predictions are confirmed by the simulations on quenched directed networks. PMID:24229156

Kwon, Sungchul; Kim, Yup

2013-10-01

158

Irreversible bimolecular chemical reactions on directed scale-free networks  

NASA Astrophysics Data System (ADS)

Kinetics of irreversible bimolecular chemical reactions A+A?0 and A+B?0 on directed scale-free networks with the in-degree distribution Pin(k)˜k-?in and the out-degree distribution Pout(?)˜?-?out are investigated. Since the correlation between k and ? of each node generally exists in directed networks, we control the correlation with the probability r?[0,1] by two different algorithms for the construction of the directed networks, i.e., the so-called k and ? algorithms. For r=1, the k algorithm gives =, whereas the ? algorithm gives =. For r=0, = for both algorithms. The kinetics of both reactions are analyzed using heterogeneous mean-field (HMF) theory and Monte Carlo simulations. The density of particles (?) algebraically decays in time t as ?(t)˜t-?. The kinetics of both reactions are determined by the same rate equation, d?/dt=a?2+b??-1, apart from coefficients. The exponent ? is determined by the algorithm: ?=?in for the k algorithm (r?0) and ?=?min for the ? algorithm (r>0), where ?min is the smaller exponent between ?in and ?out. For ?>3, one observes the ordinary mean-field kinetics, ?˜1/t (?=1). In contrast, for ?<3, ?(t) anomalously decays with ?=1/(?-2). The HMF predictions are confirmed by the simulations on quenched directed networks.

Kwon, Sungchul; Kim, Yup

2013-10-01

159

Abiotic reduction reactions of anthropogenic organic chemicals in anaerobic systems: A critical review  

NASA Astrophysics Data System (ADS)

This review is predicated upon the need for a detailed process-level understanding of factors influencing the reduction of anthropogenic organic chemicals in natural aquatic systems. In particular, abiotic reductions of anthropogenic organic chemicals are reviewed. The most important reductive reaction is alkyl dehalogenation (replacement of chloride with hydrogen) which occurs in organisms, sediments, sewage sludge, and reduced iron porphyrin model systems. An abiotic mechanism involving a free radical intermediate has been proposed. The abstraction of vicinal dihalides (also termed dehalogenation) is another reduction that may have an abiotic component in natural systems. Reductive dehalogenation of aryl halides has recently been reported and further study of this reaction is needed. Several other degradation reactions of organohalides that occur in anaerobic environments are mentioned, the most important of which is dehydrohalogenation. The reduction of nitro groups to amines has also been thoroughly studied. The reactions can occur abiotically, and are affected by the redox conditions of the experimental system. However, a relationship between nitro-reduction rate and measured redox potential has not been clearly established. Reductive dealkylation of the N- and O-heteroatom of hydrocarbon pollutants has been observed but not investigated in detail. Azo compounds can be reduced to their hydrazo derivatives and a thorough study of this reaction indicates that it can be caused by extracellular electron transfer agents. Quinone-hydroquinone couples are important reactive groups in humic materials and similar structures in resazurin and indigo carmine make them useful as models for environmental redox conditions. The interconversion of sulfones, sulfoxides, and sulfides is a redox process and is implicated in the degradation of several pesticides though the reactions need more study. Two reductive heterocyclic cleavage reactions are also mentioned. Finally, several difficulties (both semantic and experimental) that recur in the studies reviewed are discussed. The subtle effects of various sterilization techniques on extracellular biochemicals and complex chemical reducing agents in sediment have stifled attempts to separate abiotic from biological degradation reactions. The characterization of redox conditions in a natural system is still problematic since measured redox potential is not adequate. Suggestions for future research toward a process-level understanding of abiotic chemical reductions are made.

Macalady, Donald L.; Tratnyek, Paul G.; Grundl, Timothy J.

1986-02-01

160

Assessment of reaction-rate predictions of a collision-energy approach for chemical reactions in atmospheric flows.  

SciTech Connect

A recently proposed approach for the Direct Simulation Monte Carlo (DSMC) method to calculate chemical-reaction rates is assessed for high-temperature atmospheric species. The new DSMC model reproduces measured equilibrium reaction rates without using any macroscopic reaction-rate information. Since it uses only molecular properties, the new model is inherently able to predict reaction rates for arbitrary non-equilibrium conditions. DSMC non-equilibrium reaction rates are compared to Park's phenomenological nonequilibrium reaction-rate model, the predominant model for hypersonic-flow-field calculations. For near-equilibrium conditions, Park's model is in good agreement with the DSMC-calculated reaction rates. For far-from-equilibrium conditions, corresponding to a typical shock layer, significant differences can be found. The DSMC predictions are also found to be in very good agreement with measured and calculated non-equilibrium reaction rates, offering strong evidence that this is a viable and reliable technique to predict chemical reaction rates.

Gallis, Michail A.; Bond, Ryan Bomar; Torczynski, John Robert

2010-06-01

161

The quantum dynamics of electronically nonadiabatic chemical reactions  

NASA Technical Reports Server (NTRS)

Considerable progress was achieved on the quantum mechanical treatment of electronically nonadiabatic collisions involving energy transfer and chemical reaction in the collision of an electronically excited atom with a molecule. In the first step, a new diabatic representation for the coupled potential energy surfaces was created. A two-state diabatic representation was developed which was designed to realistically reproduce the two lowest adiabatic states of the valence bond model and also to have the following three desirable features: (1) it is more economical to evaluate; (2) it is more portable; and (3) all spline fits are replaced by analytic functions. The new representation consists of a set of two coupled diabatic potential energy surfaces plus a coupling surface. It is suitable for dynamics calculations on both the electronic quenching and reaction processes in collisions of Na(3p2p) with H2. The new two-state representation was obtained by a three-step process from a modified eight-state diatomics-in-molecules (DIM) representation of Blais. The second step required the development of new dynamical methods. A formalism was developed for treating reactions with very general basis functions including electronically excited states. Our formalism is based on the generalized Newton, scattered wave, and outgoing wave variational principles that were used previously for reactive collisions on a single potential energy surface, and it incorporates three new features: (1) the basis functions include electronic degrees of freedom, as required to treat reactions involving electronic excitation and two or more coupled potential energy surfaces; (2) the primitive electronic basis is assumed to be diabatic, and it is not assumed that it diagonalizes the electronic Hamiltonian even asymptotically; and (3) contracted basis functions for vibrational-rotational-orbital degrees of freedom are included in a very general way, similar to previous prescriptions for locally adiabatic functions in various quantum scattering algorithms.

Truhlar, Donald G.

1993-01-01

162

Modeling of Sheath Ion-Molecule Reactions in Plasma Enhanced Chemical Vapor Deposition of Carbon Nanotubes  

NASA Technical Reports Server (NTRS)

In many plasma simulations, ion-molecule reactions are modeled using ion energy independent reaction rate coefficients that are taken from low temperature selected-ion flow tube experiments. Only exothermic or nearly thermoneutral reactions are considered. This is appropriate for plasma applications such as high-density plasma sources in which sheaths are collisionless and ion temperatures 111 the bulk p!asma do not deviate significantly from the gas temperature. However, for applications at high pressure and large sheath voltages, this assumption does not hold as the sheaths are collisional and ions gain significant energy in the sheaths from Joule heating. Ion temperatures and thus reaction rates vary significantly across the discharge, and endothermic reactions become important in the sheaths. One such application is plasma enhanced chemical vapor deposition of carbon nanotubes in which dc discharges are struck at pressures between 1-20 Torr with applied voltages in the range of 500-700 V. The present work investigates The importance of the inclusion of ion energy dependent ion-molecule reaction rates and the role of collision induced dissociation in generating radicals from the feedstock used in carbon nanotube growth.

Hash, David B.; Govindan, T. R.; Meyyappan, M.

2004-01-01

163

Dynamic solvent effects on activated chemical reactions. I. Classical effects of reaction path curvature  

SciTech Connect

In gas phase reactions, dynamical recrossings across a phase space dividing surface induced by nonlinear reaction path curvature coupling leads to the breakdown of the fundamental dynamical approximation of classical transition state theory (TST). In the following study, we examine the nature of this breakdown for chemical reaction dynamics occurring in solution. As a model system, we consider the collinear [ital A]+[ital BC] reaction where reaction path curvature increases as the mass of [ital B] becomes small compared to the mass of [ital A] and [ital C]. We use a London--Eyring--Polanyi--Sato (LEPS) potential to describe the solute interaction and model the influence of the solvent by using a generalized Langevin equation that is further represented by a system of coupled harmonic oscillators. Exact classical rate constants are compared to those obtained from conventional TST and canonical variational transition state theory (CVT) as a function of solvent friction coupling. A harmonic TST analysis at the saddle point of the full system (solute plus solvent) with an optimum dividing surface containing both solute and solvent degrees of freedom returns the Grote--Hynes expression for the rate. For the case of no solvent coupling, both TST and CVT are identical and fail to account for the dynamical recrossings induced by reaction path curvature. At intermediate couplings, CVT provides an improvement to the TST estimate and agrees with dynamical simulation results. All estimates of the rate constant approach each other asymptotically at large couplings. The results are interpreted in terms of recrossings in the extended system (solute and solvent coordinates).

Schenter, G.K.; McRae, R.P.; Garrett, B.C. (Molecular Science Research Center, Pacific Northwest Laboratory, Richland, Washington 99352 (United States))

1992-12-15

164

Chemical Reactions in the Processing of Mosi2 + Carbon Compacts  

NASA Technical Reports Server (NTRS)

Hot-pressing of MoSi2 powders with carbon at high temperatures reduces the siliceous grain boundary phase in the resultant compact. The chemical reactions in this process were examined using the Knudsen cell technique. A 2.3 wt pct oxygen MoSi2 powder and a 0.59 wt pct oxygen MoSi2 powder, both with additions of 2 wt pct carbon, were examined. The reduction of the siliceous grain boundary phase was examined at 1350 K and the resultant P(SiO)/P(CO) ratios interpreted in terms of the SiO(g) and CO(g) isobars on the Si-C-O predominance diagram. The MoSi2 + carbon mixtures were then heated at the hot-pressing temperature of 2100 K. Large weight losses were observed and could be correlated with the formation of a low-melting eutectic and the formation and vaporization of SiC.

Jacobson, Nathan S.; Lee, Kang N.; Maloy, Stuart A.; Heuer, Arthur H.

1993-01-01

165

Measurements and analysis of alpha-induced reactions of importance for nuclear astrophysics  

NASA Astrophysics Data System (ADS)

Reactions during stellar helium burning are of primary importance for understanding nucleosynthesis. A detailed understanding of the critical reaction chain 4He(2alpha, gamma)12C( alpha, gamma)16O(alpha, gamma) 20Ne is necessary both because it is the primary energy source and because it determines the ratio of 12C to 16O produced, which in turn significantly effects subsequent nucleosynthesis. Also during Helium burning, the reactions 22Ne(alpha, n)25Mg and 22Ne(alpha, gamma )26Mg are crucial in determining the amount of neutrons available for the astrophysical s-process. This thesis presents new experimental results concerning the 16O(alpha, gamma) 20Ne, 22Ne(alpha, n)25Mg, and 22Ne(alpha, gamma)26Mg reaction rates. These results are then applied to the calculation of the associated stellar reaction rates in order to achieve better accuracy.

de Messieres, Genevieve Escande

2011-11-01

166

Scaling laws for pulsed chain-reaction chemical lasers  

NASA Astrophysics Data System (ADS)

Scaling laws for pulsed chain-reaction chemical lasers are deduced with the use of a two-level vibrational model. The performance of a saturated laser depends only on the parameter K = t sub cd/t sub p, where t sub cd and t sub p are the characteristic collisional deactivation and characteristic pumping times, respectively. The normalized output energy per unit volume per pulse of a saturated HF chain-reaction laser is 2E/Epsilon H sub 2,0 = K(1 + 0(K)), where E is output energy per unit volume per pulse, Epsilon is energy per mole of photons, and H sub 2, 0 is the initial concentration of H sub 2 in moles per unit volume. In the range 0.02 < or = thi << 1 the normalized output energy from a saturated HF laser can be expressed as 2E/Epsilon H sub 2, 0 = thi, where thi approx. = (F/F sub 2) sub 0 1/2(F sub 2/H sub 2)0(1 + 0.094(F sub 2/H sub 2) sub 0) to the minus 1/2 power. In the latter regime the product Et sub e is a constant for a saturated laser (t sub e = pulse length). Corrections for multiple vibrational levels are given in an Appendix.

Mirels, H.; Hofland, R., Jr.; Whittier, J. S.

1981-08-01

167

Kinetics of Thermochemical Reactions Important in the Venus Atmospheric Sulfur Cycle  

NASA Technical Reports Server (NTRS)

The purpose of this project was to experimentally measure the rates of several thermochemical gas-solid reactions between sulfur gases in the Venus atmosphere and reactive minerals on the hot Venus surface. Despite the great importance of these reactions for the maintenance of significant amounts of sulfur gases (and thus for the maintenance of the global cloud cover) in the atmosphere of Venus, essentially no kinetic data are currently available for them.

Fegley, Bruce, Jr.

1997-01-01

168

Chloroaluminum phthalocyanine thin films: chemical reaction and molecular orientation.  

PubMed

The chemical transformation of the polar chloroaluminum phthalocyanine, AlClPc, to ?-(oxo)bis(phthalocyaninato)aluminum(III), (PcAl)2O, in thin films on indium tin oxide is studied and its influence on the molecular orientation is discussed. The studies were conducted using complementary spectroscopic techniques: Raman spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. In addition, density functional theory calculations were performed in order to identify specific vibrations and to monitor the product formation. The thin films of AlClPc were annealed in controlled environmental conditions to obtain (PcAl)2O. It is shown that the chemical transformation in the thin films can proceed only in the presence of water. The influence of the reaction and the annealing on the molecular orientation was studied with Raman spectroscopy and NEXAFS spectroscopy in total electron yield and partial electron yield modes. The comparison of the results obtained from these techniques allows the determination of the molecular orientation of the film as a function of the probing depth. PMID:23494276

Latteyer, Florian; Peisert, Heiko; Uihlein, Johannes; Basova, Tamara; Nagel, Peter; Merz, Michael; Schuppler, Stefan; Chassé, Thomas

2013-05-01

169

A Comprehensive Chemical Kinetic Reaction Mechanism for Oxidation and Pyrolysis of Propane and Propene  

Microsoft Academic Search

Abstract—A detailed chemical kinetic reaction mechanism is developed to describe the oxidation and pyrolysis of propane and propene. The mechanism consists of 163 elementary reactions among 4l chemical species. New rate expressions are developed for a number of reactions of propane, propene, and intermediate hydrocarbon species with radicals including H, 0, and OH. The mechanism is tested by comparisons between

CHARLES K. WESTBROOK; WILLIAM J. PITZ

1984-01-01

170

MODEL OF CHEMICAL REACTION EQUILIBRIUM OF SULFURIC ACID SALTS OF TRIOCTYLAMINE  

Microsoft Academic Search

The chemical reaction of trioctylamine (TOA) and sulfuric acid in organic solvent\\/aqueous solution was carried out. TOA salt products of various kinds were obtained based on different conditions of operation and organic solvents. An equilibrium model, based on the chemical reaction of sulfuric acid and trioctylamine, is proposed. The equilibrium constants of various reactions of trioctylamine and sulfuric acid were

MAW-LING WANG; KWAN-HUA HU

1993-01-01

171

Transport Properties of a Kinetic Model for Chemical Reactions without Barriers  

SciTech Connect

A kinetic model of the Boltzmann equation for chemical reactions without energy barrier is considered here with the aim of evaluating the reaction rate and characterizing the transport coefficient of shear viscosity for the reactive system. The Chapman-Enskog solution of the Boltzmann equation is used to compute the chemical reaction effects, in a flow regime for which the reaction process is close to the final equilibrium state. Some numerical results are provided illustrating that the considered chemical reaction without energy barrier can induce an appreciable influence on the reaction rate and on the transport coefficient of shear viscosity.

Alves, Giselle M. [Escola Tecnica, Universidade Federal do Parana, Curitiba (Brazil); Kremer, Gilberto M. [Departamento de Fisica, Universidade Federal do Parana, Curitiba (Brazil); Soares, Ana Jacinta [Departamento de Matematica, Universidade do Minho, Braga (Portugal)

2011-05-20

172

Chemical Kinetic Reaction Mechanisms for Combustion of Hydrocarbon and Other Types of Chemical Fuels  

DOE Data Explorer

The central feature of the Combustion Chemistry project at LLNL is the development, validation, and application of detailed chemical kinetic reaction mechanisms for the combustion of hydrocarbon and other types of chemical fuels. For the past 30 years, LLNL's Chemical Sciences Division has built hydrocarbon mechanisms for fuels from hydrogen and methane through much larger fuels including heptanes and octanes. Other classes of fuels for which models have been developed include flame suppressants such as halons and organophosphates, and air pollutants such as soot and oxides of nitrogen and sulfur. Reaction mechanisms have been tested and validated extensively through comparisons between computed results and measured data from laboratory experiments (e.g., shock tubes, laminar flames, rapid compression machines, flow reactors, stirred reactors) and from practical systems (e.g., diesel engines, spark-ignition engines, homogeneous charge, compression ignition (HCCI) engines). These kinetic models are used to examine a wide range of combustion systems.[Taken from https://www-pls.llnl.gov/?url=science_and_technology-chemistry-combustion

173

Chemical reaction pathways affecting stratospheric and mesospheric ozone  

NASA Astrophysics Data System (ADS)

Catalytic cycles and other chemical pathways affecting ozone are normally estimated empirically in atmospheric models. In this work we have automatically quantified such processes by applying a newly developed analysis package called the "Pathway Analysis Program" (PAP). It used modeled chemical rates and concentrations as input. These were supplied by the "Module Efficiently Calculating the Chemistry of the Atmosphere" MECCA box model, itself initialized by the Free University of Berlin Climate Middle Atmosphere Model with Chemistry. We analyzed equatorial, midlatitude and high-latitude locations over 24-hour periods during spring in both hemispheres. We present results for locations in the lower stratosphere, upper stratosphere and midmesosphere. Oxygen photolysis dominated (>99%) in situ ozone production in the equatorial lower stratosphere, in the upper stratosphere and in the mesosphere. In the lower stratosphere midlatitudes the "ozone smog cycle" (already established in the troposphere) rivaled oxygen photolysis as an in situ ozone source in both hemispheres. However, absolute ozone production rates in midlatitudes were rather slow compared with at the equator, typically 16-50 ppt ozone/day. In the equatorial lower stratosphere, five catalytic sinks were important (each contributing at least 5% to chemical ozone loss): a HOx cycle, a HOBr cycle and its HOCl analog, a water-HOx cycle and a mixed chlorine-bromine cycle. Important in midlatitudes were the HOx cycle, a NOx cycle, the HOBr cycle and the mixed chlorine-bromine cycle. In lower-stratosphere high latitudes, the chlorine dimer cycle and the mixed chlorine-bromine cycle dominated in both hemispheres. A variant on the latter, involving BrCl formation, also featured. In the upper stratosphere high latitudes (where strong negative ozone trends are observed), a nitrogen cycle, a chlorine cycle, and a mixed chlorine-nitrogen cycle were found. In the mesosphere, three closely related HOx cycles dominated ozone loss.

Grenfell, J. Lee; Lehmann, Ralph; Mieth, Peter; Langematz, Ulrike; Steil, Benedikt

2006-09-01

174

Chemical Characterization of Secondary Organic Aerosol Formed from Atmospheric Aqueous-phase Reactions of Phenolic Compounds  

NASA Astrophysics Data System (ADS)

Phenolic compounds, which are released in significant amounts from biomass burning, may undergo fast aqueous-phase reactions to form secondary organic aerosol (SOA) in the atmosphere. Understanding the aqueous-phase reaction mechanisms of these compounds and the composition of their reaction products is thus important for constraining SOA sources and predicting organic aerosol properties in models. In this study, we investigate the aqueous-phase reactions of three phenols (phenol, guaiacol and syringol) with two oxidants - excited triplet states (3C*) of non-phenolic aromatic carbonyls and hydroxyl radical (OH). By employing four analytical methods including high-resolution aerosol mass spectrometry, total organic carbon analysis, ion chromatography, and liquid chromatography-mass spectrometry, we thoroughly characterize the chemical compositions of the low volatility reaction products of phenols and propose formation mechanisms based on this information. Our results indicate that phenolic SOA is highly oxygenated, with O/C ratios in the range of 0.83-1.03, and that the SOA of phenol is usually more oxidized than those of guaiacol and syringol. Among the three precursors, syringol generates the largest fraction of higher molecular weight (MW) products. For the same precursor, the SOA formed via reaction with 3C* is less oxidized than that formed via reaction with OH. In addition, oxidation by 3C* enhances the formation of higher MW species, including phenolic dimers, higher oligomers and hydroxylated products, compared to reactions initiated by OH, which appear to favor the formation of organic acids. However, our results indicate that the yields of small organic acids (e.g., formate, acetate, oxalate, and malate) are low for both reaction pathways, together accounting for less than 5% of total SOA mass.

Yu, L.; Smith, J.; Anastasio, C.; Zhang, Q.

2012-12-01

175

Relaxation of chemical reactions to stationary states in the chemical affinities space  

NASA Astrophysics Data System (ADS)

Using the mass balance equations for chemical reactions, we show how the system relaxes toward a steady-state in and out of the Onsager region. In the chemical affinities space, after fast transients, the relaxation process is a straight line when operating in the Onsager region, while out of this regime, the evolution of the system is such that the projections of the evolution equations for the forces and the shortest path on the flows coincide. For spatially extended systems, similar results are valid for the evolution of the thermodynamic mode (i.e., the mode with wave-number k = 0). These results allow us to obtain the expression for the affine connection of the space covered by the thermodynamic forces, close to the steady-states. Through the affine connection, the nonlinear closure equations are derived.

Sonnino, Giorgio

2010-12-01

176

No electron left behind: a rule-based expert system to predict chemical reactions and reaction mechanisms.  

PubMed

Predicting the course and major products of arbitrary reactions is a fundamental problem in chemistry, one that chemists must address in a variety of tasks ranging from synthesis design to reaction discovery. Described here is an expert system to predict organic chemical reactions based on a knowledge base of over 1500 manually composed reaction transformation rules. Novel rule extensions are introduced to enable robust predictions and describe detailed reaction mechanisms at the level of electron flows in elementary reaction steps, ensuring that all reactions are properly balanced and atom-mapped. The core reaction prediction functionalities of this expert system are illustrated with applications including: (1) prediction of detailed reaction mechanisms; (2) computer-based learning in organic chemistry; (3) retrosynthetic analysis; and (4) combinatorial library design. Select applications are available via http://cdb.ics.uci.edu. PMID:19719121

Chen, Jonathan H; Baldi, Pierre

2009-09-01

177

Theoretical study on the mechanism and kinetics of acetaldehyde and hydroperoxyl radical: An important atmospheric reaction  

NASA Astrophysics Data System (ADS)

A systematic theoretical study was performed on the mechanism and kinetics of the atmospheric reaction of acetaldehyde (CH3CHO) and hydroperoxyl radical (HO2) in the gas phase. The DFT-B3LYP/6-311++G(3df,3pd) and CCSD(T)/6-311++G(d,p) methods were employed for calculations. Based on the calculations, this reaction leads to four different products through radical addition and hydrogen abstraction mechanisms which are very important in atmospheric and combustion chemistry. The favorable reaction paths begin with ?-hydroxyethylperoxy radical, CH3CH(OO)OH, in a exothermic process and finally leads to the product P1 (CH3COOH + OH). The overall rate constants for favorite reaction paths have been calculated at different temperatures (200-2500 K).

Farnia, Solaleh; Vahedpour, Morteza; Abedi, Mostafa; Farrokhpour, Hossein

2013-09-01

178

A Two-Photon Induced Chemical Reaction in AgCl.  

National Technical Information Service (NTIS)

Two photon absorptions have been observed in a variety of materials. The only reported instances of the initiation of chemical reactions by such an absorption are polymerization reactions of organic molecules. We wish to report the two photon absorption b...

D. L. Rousseau G. E. Leroi G. L. Link

1965-01-01

179

RPMDRATE: Bimolecular chemical reaction rates from ring polymer molecular dynamics  

NASA Astrophysics Data System (ADS)

We present RPMDRATE, a computer program for the calculation of gas phase bimolecular reaction rate coefficients using the ring polymer molecular dynamics (RPMD) method. The RPMD rate coefficient is calculated using the Bennett-Chandler method as a product of a static (centroid density quantum transition state theory (QTST) rate) and a dynamic (ring polymer transmission coefficient) factor. The computational procedure is general and can be used to treat bimolecular polyatomic reactions of any complexity in their full dimensionality. The program has been tested for the H+H2, H+CH4, OH+CH4 and H+C2H6 reactions. Program summaryProgram title: RPMDrate Catalogue identifier: AENW_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AENW_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: MIT license No. of lines in distributed program, including test data, etc.: 94512 No. of bytes in distributed program, including test data, etc.: 1395674 Distribution format: tar.gz Programming language: Fortran 90/95, Python (version 2.6.x or later, including any version of Python 3, is recommended). Computer: Not computer specific. Operating system: Any for which Python, Fortran 90/95 compiler and the required external routines are available. Has the code been vectorized or parallelized?: The program can efficiently utilize 4096+ processors, depending on problem and available computer. At low temperatures, 110 processors are reasonable for a typical umbrella integration run with an analytic potential energy function and gradients on the latest x86-64 machines. RAM: 256 Mb Classification: 16.12. External routines: NumPy (http://numpy.scipy.org, version 1.5.0 or later is recommended). FFTW3 (http://www.fftw.org, version 3.3 or later is recommended). Nature of problem: The RPMDrate program calculates thermal bimolecular rate coefficients of thermally activated atom-diatom and more complex bimolecular chemical reactions in the gas phase. Solution method: The RPMD rate is calculated using the Bennett-Chandler factorization as a product of a static (centroid density quantum transition state theory (QTST) rate) and a dynamic (transmission coefficient) factor. A key feature of this procedure is that it does not require that one calculate the absolute quantum mechanical partition function of the reactants or the transition state. The centroid density QTST rate is calculated from the potential of mean force along the reaction coordinate using umbrella integration. The reaction coordinate is taken to be an interpolating function that connects two dividing surfaces: one located in the asymptotic reactant valley and the other located in the transition state region. The Hessian of the collective reaction coordinate is obtained analytically. The transmission coefficient is calculated from the RPMD simulations with the hard constraint along the reaction coordinate. Restrictions: The applicability of RPMDrate is restricted to global potential energy surfaces with gradients. In the current release, they should be provided by Python callable objects. Unusual features: Simple and user-friendly input system provided by Python syntax. Additional comments: Test calculations for the H+H2 reactions were performed using the Boothroyd-Keogh-Martin-Peterson-2 (BKMP2) potential energy surface (PES) [1]. PESs for the H+CH4,OH+CH4 and H+C2H6 reactions are taken from the online POTLIB library [2]. PESs are included within the distribution package as Fortran subroutines. Implementations of the colored-noise, generalized Langevin equation (GLE) thermostats [3-5] have been included in the current release. The distribution contains example data and a detailed manual describing the use of RPMDrate. Running time: Highly dependent on the input parameters. The running time of RPMDrate depends mainly on the complexity of the potential energy surface and number of ring polymer beads. For the H+H2, H+CH4, and OH+CH4 test calculations given (with 128 ring polymer beads and analytic gradients), the running time is

Suleimanov, Yu. V.; Allen, J. W.; Green, W. H.

2013-03-01

180

Beyond transition state theory: Rigorous quantum approaches for determining chemical reaction rates  

SciTech Connect

Transition state theory (TST) has historically been the most important and widely used theoretical approach for describing the rates of chemical reactions, and for qualitative pictures and order-of-magnitude estimates one does not expect this situation to change. However a rigorous, quantitative treatment of chemical reaction rates must go beyond TST. A rigorous description, for example, must be based on a quantum mechanical description of the molecular system, but the fundamental assumption on which TST is based - namely that the molecular dynamics is {open_quotes}direct,{close_quotes} i.e., that no trajectories re-cross a dividing surface which separates reactants and products (vide infra) - is couched inherently in the language of classical mechanics. There is no unambiguous way to quantize TST, for the various ways of trying to do so invariably require one to introduce additional assumptions about the reaction dynamics. As one tries to eliminate these {open_quotes}additional assumptions{close_quotes} one is driven ultimately to an exact quantum treatment of the reaction dynamics which is then no longer a transition state theory (i.e., approximation) but simply an exact formulation. It is such exact approaches, those without inherent approximations, that are the subject of this chapter.

Miller, W.H.

1995-01-01

181

Detection of a wide range of medically important fungi by the polymerase chain reaction  

Microsoft Academic Search

Summary. A polymerase chain reaction (PCR) method was developed that was capable of detecting a wide range of medically important fungi from clinical specimens. The primer pair was designed in conserved sequences of 1 8s-ribosomal RNA genes shared by most fungi. The lower limit of detection of this PCR technique was 1 pg of Cundidu ulbicans genomic DNA by ethidium

K. Makimura; SOMAY Y. MURAYAMA; H. Yamaguchi

1994-01-01

182

Fatty acid oxidation in anoxic marine sediments: the importance of hydrogen sensitive reactions  

Microsoft Academic Search

In anoxic marine sediments fatty acids may be oxidized directly by sulfate reducing bacteria, or may be oxidized by pathways which result in hydrogen production. Some of these latter reactions are quite sensitive to hydrogen concentrations ... in other words if hydrogen concentrations become elevated, fatty acid oxidation will cease. Thus sulfate reducers may actually play two important roles in

Matthew A. Monetti; Mary I. Scranton

1992-01-01

183

Final Technical Report "Energy Partitioning in Elementary Chemical Reactions"  

SciTech Connect

This is the final technical report of the subject grant. It describes the scientific results obtained during the reporting period. These results are focused on the reactions of atomic oxygen with terminal alkenes. We have studied the production of vinoxy in these reactions. We have characterized the energy disposal in the reactions and have elaborated the reaction mechanism.

Richard Bersohn (deceased); James J. Valentini (reporting investigator)

2005-10-03

184

Mathematically Reduced Chemical Reaction Mechanism Using Neural Networks  

SciTech Connect

This is an annual technical report for the work done over the last year (period ending 9/30/2004) on the project titled ''Mathematically Reduced Chemical Reaction Mechanism Using Neural Networks''. The aim of the project is to develop an efficient chemistry model for combustion simulations. The reduced chemistry model will be developed mathematically without the need of having extensive knowledge of the chemistry involved. To aid in the development of the model, Neural Networks (NN) will be used via a new network topology know as Non-linear Principal Components Analysis (NPCA). We report on the development of a procedure to speed up the training of NPCA. The developed procedure is based on the non-parametric statistical technique of kernel smoothing. When this smoothing technique is implemented as a Neural Network, It is know as Generalized Regression Neural Network (GRNN). We present results of implementing GRNN on a test problem. In addition, we present results of an in house developed 2-D CFD code that will be used through out the project period.

Nelson Butuk

2004-12-01

185

Chemical reactions between Venus' surface and atmosphere - An update. (Invited)  

NASA Astrophysics Data System (ADS)

The surface of Venus, at ~740K, is hot enough to allow relatively rapid chemical reactions between it and the atmosphere, i.e. weathering. Venus chemical weathering has been explored in detail [1], to the limits of available data. New data from Venus Express (VEx) and new ideas from exoplanets have sparked a modest renewal of interest in Venus weathering. Venus' surface cannot be observed in visible light, but there are several NIR ';windows' through its atmosphere that allow surface imaging. The VIRTIS spectrometer on VEx viewed the surface through one window [2]; emissivity variations among lava flows on Imdr and Themis Regios have been explained as varying degrees of weathering, and thus age [3]. The VMC camera on VEx also provides images through a NIR window, which suggest variable degrees of weathering on some basaltic plains [4]. Indirect evidence for weathering may come from varying SO2 abundance at Venus' cloud tops; repeated rapid increases and gradual declines may represent volcanic eruptions followed by weathering to form sulfate minerals [5]. Continued geochemical modeling relevant to Venus weathering is motivated by expolanet studies [6]. Models have been extended to hypothetical exo-Venuses of different temperatures and surface compositions [7]. The idea that Venus' atmosphere composition can be buffered by reaction with its surface was explored in detail, and the derived constraint extended to other types of planets [8]. Several laboratories are investigating Venus weathering, motivated in part by the hope that they can provide real constraints on timescales of Venus volcanism [3]. Aveline et al. [9] are extending early studies [10] by reacting rocks and minerals with concentrated SO2 (to accelerate reaction rates to allow detectability of products). Kohler et al. [11] are investigating the stability of metals and chalcogenides as possible causes of the low-emissivity surfaces at high elevations. Berger and Aigouy [12] studied rock alteration on a hypothetical early Venus with a water-rich atmosphere. Martin et al. [13] investigated the fate of weathered rock when heated (by igneous or impact events). Our understanding of Venus' geological history is stymied by a lack of data - spacecraft observations of and/or at its surface. VMC on VEx may continue to provide new data on surface emissivity, but their interpretation is inherently ambiguous. Laboratory experiments seem the most promising approach - attempting to quantify rates of weathering and thus volcanism [3], and (with luck) framing significant problems that can be directly answered by spacecraft observations. [1] Fegley B.Jr. et al. (1997) In Venus II. U. Ariz. Press. p. 591. [2] Helbert J. et al. (2008) GRL 35, L11201. [3] Smrekar S.E et al. (2010) Science 328, 605-608. [4] Basilevsky A.T. et al. (2012) Icarus 217, 434-450. [5] Marcq E. et al. (2013) Nature Geoscience 6, 25-28. [6] Kane S.R. et al. (2013) Astrophysical J. 770, L20. [7] Schaefer L. & Fegley B.Jr. (2011) Astrophysical J. 729, 6. [8] Treiman A.H. & Bullock M.A. (2012) Icarus 217, 534-541. [9] Aveline D.C. et al. (2011) Lunar Planet. Sci. Conf. 42, Abstr. #2165. [10] Fegley B.Jr. & Prinn R.G. (1989) Nature 337, 55-58. [11] Kohler E. et al. (2012) Lunar Planet. Sci. Conf. 43, Abstr. #2749. [12] Berger G. & Aigouy T. (2011) Lunar Planet. Sci. Conf. 42, Abstr. #1660. [13] Martin A.M. et al. (2012) Earth Planet. Sci. Lett. 331-332, 291-304.

Treiman, A. H.

2013-12-01

186

Detailed Chemical Kinetic Reaction Mechanisms for Autoignition of Isomers of Heptane Under Rapid Compression  

SciTech Connect

Detailed chemical kinetic reaction mechanisms are developed for combustion of all nine isomers of heptane (C{sub 7}H{sub 16}), and these mechanisms are tested by simulating autoignition of each isomer under rapid compression machine conditions. The reaction mechanisms focus on the manner in which the molecular structure of each isomer determines the rates and product distributions of possible classes of reactions. The reaction pathways emphasize the importance of alkylperoxy radical isomerizations and addition reactions of molecular oxygen to alkyl and hydroperoxyalkyl radicals. A new reaction group has been added to past models, in which hydroperoxyalkyl radicals that originated with abstraction of an H atom from a tertiary site in the parent heptane molecule are assigned new reaction sequences involving additional internal H atom abstractions not previously allowed. This process accelerates autoignition in fuels with tertiary C-H bonds in the parent fuel. In addition, the rates of hydroperoxyalkylperoxy radical isomerization reactions have all been reduced so that they are now equal to rates of analogous alkylperoxy radical isomerizations, significantly improving agreement between computed and experimental ignition delay times in the rapid compression machine. Computed ignition delay times agree well with experimental results in the few cases where experiments have been carried out for specific heptane isomers, and predictive model calculations are reported for the remaining isomers. The computed results fall into three general groups; the first consists of the most reactive isomers, including n-heptane, 2-methyl hexane and 3-methyl hexane. The second group consists of the least reactive isomers, including 2,2-dimethyl pentane, 3,3-dimethyl pentane, 2,3-dimethyl pentane, 2,4-dimethyl pentane and 2,2,3-trimethyl butane. The remaining isomer, 3-ethyl pentane, was observed computationally to have an intermediate level of reactivity. These observations are generally consistent with knocking tendencies of these isomers, as measured by octane ratings, although the correlations are only approximate.

Westbrook, C K; Pitz, W J; Boercker, J E; Curran, H J; Griffiths, J F; Mohamed, C; Ribaucour, M

2001-12-17

187

Optomechanical manipulation of chemical reactions on the nanoscale with optofluidic nanotweezers  

NASA Astrophysics Data System (ADS)

Chemical reactions are often described as a progression along a reaction coordinate. Waveguide evanescent fields generate an electromagnetic force that spans tens of nanometers and have been used previously to trap protein molecules. Applying this force along a reaction coordinate could radically alter the chemical reaction by modifying the activation energy or biasing the reaction towards a specific pathway. Here, we show that the adsorption of proteins onto carbon nanotubes can be controlled with opto-mechanical forces. An analytic model for the reaction was developed, the predictions of which were explored by probing the energy barrier under various experimental conditions.

O'Dell, Dakota; Serey, Xavier; Erickson, David

2014-03-01

188

Oxidation of NaBr Aerosol by Ozone and Hydroxyl Radicals: Importance of Reactions  

NASA Astrophysics Data System (ADS)

The release of Br atoms from photolyzable bromine species is responsible for the almost complete destruction of ground-level ozone observed in the Arctic after Polar sunrise, and likely for the partial destruction of ozone observed in the marine boundary layer at mid-latitudes. Based on previous studies of the reaction of deliquesced NaCl particles with hydroxyl radicals, a surface reaction mechanism was proposed to explain formation of photolytically active Cl2. A similar reaction producing Br2 should be more rapid in bromide-containing aerosols where the bromide ion concentration is expected to be enhanced at the gas-particle interface. To investigate the mechanism of the reactions of ozone and hydroxyl with NaBr aerosol, experiments were carried out at room temperature and atmospheric pressure in a 561 L aerosol chamber at relative humidity above the deliquescence point of NaBr aerosol. Fourier transform infrared spectroscopy (FTIR) and differential optical absorption spectroscopy (DOAS) were used to measure the concentrations of ozone and the intermediate species BrO. The concentration of Br2 was monitored using atmospheric pressure chemical ionization mass spectrometry. Additionally, the composition of individual aerosol particles collected from the chamber was analyzed with computer controlled scanning electron microscopy with energy-dispersed analysis of x-rays (CCSEM/EDX) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS). Analysis of particles collected after reaction with ozone and hydroxyl revealed bromine depletion and oxygen enrichment. The mechanism for bromine production was evaluated with a computer kinetics box model that includes gas and aqueous phase chemical reactions, gas and aqueous phase diffusion, and mass transfer between the liquid aerosol droplets and the gas phase. The modeling showed that known gas phase and aqueous phase bromine chemistry alone could not reproduce experimental results. However, with the inclusion of a reaction at the air-water interface between gaseous ozone and aqueous bromide ion, the model reproduces experimental results for bromine production reasonably well. The atmospheric implications of this type heterogeneous chemistry at interfaces will be discussed.

Hunt, S. W.; Wang, W.; Laskin, A.; Gaspar, D. J.; Wingen, L. M.; Finlayson-Pitts, B. J.

2003-12-01

189

Computed Potential Energy Surfaces and Minimum Energy Pathways for Chemical Reactions  

NASA Technical Reports Server (NTRS)

Computed potential energy surfaces are often required for computation of such parameters as rate constants as a function of temperature, product branching ratios, and other detailed properties. For some dynamics methods, global potential energy surfaces are required. In this case, it is necessary to obtain the energy at a complete sampling of all the possible arrangements of the nuclei, which are energetically accessible, and then a fitting function must be obtained to interpolate between the computed points. In other cases, characterization of the stationary points and the reaction pathway connecting them is sufficient. These properties may be readily obtained using analytical derivative methods. We have found that computation of the stationary points/reaction pathways using CASSCF/derivative methods, followed by use of the internally contracted CI method to obtain accurate energetics, gives usefull results for a number of chemically important systems. The talk will focus on a number of applications including global potential energy surfaces, H + O2, H + N2, O(3p) + H2, and reaction pathways for complex reactions, including reactions leading to NO and soot formation in hydrocarbon combustion.

Walch, Stephen P.; Langhoff, S. R. (Technical Monitor)

1994-01-01

190

A reaction-based paradigm to model reactive chemical transport in groundwater with general kinetic and equilibrium reactions  

NASA Astrophysics Data System (ADS)

This paper presents a reaction-based water quality transport model in subsurface flow systems. Transport of chemical species with a variety of chemical and physical processes is mathematically described by M partial differential equations (PDEs). Decomposition via Gauss-Jordan column reduction of the reaction network transforms M species reactive transport equations into two sets of equations: a set of thermodynamic equilibrium equations representing NE equilibrium reactions and a set of reactive transport equations of M- NE kinetic-variables involving no equilibrium reactions (a kinetic-variable is a linear combination of species). The elimination of equilibrium reactions from reactive transport equations allows robust and efficient numerical integration. The model solves the PDEs of kinetic-variables rather than individual chemical species, which reduces the number of reactive transport equations and simplifies the reaction terms in the equations. A variety of numerical methods are investigated for solving the coupled transport and reaction equations. Simulation comparisons with exact solutions were performed to verify numerical accuracy and assess the effectiveness of various numerical strategies to deal with different application circumstances. Two validation examples involving simulations of uranium transport in soil columns are presented to evaluate the ability of the model to simulate reactive transport with complex reaction networks involving both kinetic and equilibrium reactions.

Zhang, Fan; Yeh, Gour-Tsyh; Parker, Jack C.; Brooks, Scott C.; Pace, Molly N.; Kim, Young-Jin; Jardine, Philip M.; Watson, David B.

2007-06-01

191

Detailed Chemical Kinetic Reaction Mechanisms for Incineration of Organophosphorus and Fluoro-Organophosphorus Compounds  

Microsoft Academic Search

A detailed chemical kinetic reaction mechanism is developed to describe incineration of the chemical warfare nerve agent sarin (GB), based on commonly used principles of bond additivity and hierarchical reaction mechanisms. The mechanism is based on previous kinetic models of organophosphorus compounds such as TMP, DMMP and DIMP that are often used as surrogates to predict incineration of GB. Kinetic

P A Glaude; C Melius; W J Pitz; C K Westbrook

2001-01-01

192

Application of the shock tube unsteady expansion wave technique to the study of chemical reactions  

Microsoft Academic Search

An unsteady expansion wave was generated through rupture of a secondary diaphragm by an incident shock wave in a shock tube to study the three-body recombination of iodine atoms. The application to chemical reaction studies has been made possible through an extension of the usable flow time, and a theoretical treatment enables the effect of coupling of chemical reaction to

W. H. Beck; J. C. Mackie

1978-01-01

193

Acid-Base Chemistry According to Robert Boyle: Chemical Reactions in Words as well as Symbols  

ERIC Educational Resources Information Center

Examples of acid-base reactions from Robert Boyle's "The Sceptical Chemist" are used to illustrate the rich information content of chemical equations. Boyle required lengthy passages of florid language to describe the same reaction that can be done quite simply with a chemical equation. Reading or hearing the words, however, enriches the student's…

Goodney, David E.

2006-01-01

194

Method of operating a thermal engine powered by a chemical reaction  

DOEpatents

The invention involves a novel method of increasing the efficiency of a thermal engine. Heat is generated by a non-linear chemical reaction of reactants, said heat being transferred to a thermal engine such as Rankine cycle power plant. The novel method includes externally perturbing one or more of the thermodynamic variables of said non-linear chemical reaction. 7 figs.

Ross, J.; Escher, C.

1988-06-07

195

Influence of Chemical Reactions on the Mobility of Radionuclides in the Terrestrial Environment.  

National Technical Information Service (NTIS)

The influence of the kinetics of chemical and physico-chemical reactions on the mobility of radionuclides in the terrestrial environment has been studied theoretically by application of the COLUMN2 computer code. A variety of reaction systems has been ana...

L. Carlsen O. J. Nielsen P. Bo C. Ditlevsen

1986-01-01

196

Design criteria for extraction with chemical reaction and liquid membrane permeation  

NASA Technical Reports Server (NTRS)

The design criteria for heterogeneous chemical reactions in liquid/liquid systems formally correspond to those of classical physical extraction. More complex models are presented which describe the material exchange at the individual droplets in an extraction with chemical reaction and in liquid membrane permeation.

Bart, H. J.; Bauer, A.; Lorbach, D.; Marr, R.

1988-01-01

197

CHEMICAL SYNTHESIS USING 'GREENER' ALTERNATIVE REACTION CONDITIONS AND MEDIA  

EPA Science Inventory

The chemical research during the last decade has witnessed a paradigm shift towards "environmentally-friendly chemistry" more popularly known as "green chemistry" due to the increasing environmental concerns and legislative requirements to curb the release of chemical waste into ...

198

Heat-of-Reaction Chemical Heat Pumps: Possible Configurations.  

National Technical Information Service (NTIS)

Chemical heat pumps utilize working fluids which undergo reversible chemical changes. Mechanically driven reactive heat pump cycles or, alternatively, heat driven heat pumps in which either heat engine or heat pump working fluid is reactive, are considere...

L. D. Kirol

1986-01-01

199

NEURAL - a tracking detector for neutron-induced reactions of astrophysical importance  

NASA Astrophysics Data System (ADS)

Observations from gamma ray telescopes indicate that most of the galactic 26Al originates in massive stars. Several sites have been discussed for its production, including proton burning in the winds of very massive stars, and the later, explosive burning stages of these stars. Observations from the RHESSI and INTEGRAL missions currently seem to point to the latter scenario. In the advanced burning stages of massive stars the presence of neutrons becomes an important factor in nuclear reaction networks, so in addition to the 26Al(p,?)27Si reaction the neutron capture reactions 26Al(n,p)26Mg and 26Al(n,?)23Na can lead to the destruction of 26Al, and thus alter the observed 26Al abundance. NEURAL is a detector design to measure the excitation functions of these reactions over a wide range of energies. 26Al targets implanted at TRIUMF will be exposed to a pulsed neutron beam at the neutron time-of-flight facility at LANSCE (Los Alamos Neutron Science CEnter). NEURAL is designed to detect all charged reaction products, combining a time projection chamber for the heavy ions, and Si detectors for the light particles mounted around the target. A first prototype has been built and partially tested at TRIUMF and LANSCE in December 2009.

Martin, L.; Buchmann, L.; Carpino, J. F.; Chen, A.; Couture, A.; Davids, B.; Fallis, J.; Fox, S. P.; Hager, U.; Hussein, A.; Laird, A. M.; Olchanski, K.; Ottewell, D.; Ruiz, C.; Ruprecht, G.; Sheffer, G.; Sjue, S.; Tengblad, O.; Tovesson, F.

2011-09-01

200

Polarization of molecular angular momentum in the chemical reactions Li + HF and F + HD  

NASA Astrophysics Data System (ADS)

The quantum mechanical approach to vector correlation of angular momentum orientation and alignment in chemical reactions [G. Balint-Kurti and O. S. Vasyutinskii, J. Phys. Chem. A 113, 14281 (2009)] is applied to the molecular reagents and products of the Li + HF [L. Gonzalez-Sanchez, O. S. Vasyutinskii, A. Zanchet, C. Sanz-Sanz, and O. Roncero, Phys. Chem. Chem. Phys. 13, 13656 (2011)] and F + HD [D. De Fazio, J. Lucas, V. Aquilanti, and S. Cavalli, Phys. Chem. Chem. Phys. 13, 8571 (2011)] reactions for which accurate scattering information has become recently available through time-dependent and time-independent approaches. Application of the theory to two important particular cases of the reactive collisions has been considered: (i) the influence of the angular momentum polarization of reactants in the entrance channel on the spatial distribution of the products in the exit channel and (ii) angular momentum polarization of the products of the reaction between unpolarized reactants. In the former case, the role of the angular momentum alignment of the reactants is shown to be large, particularly when the angular momentum is perpendicular to the reaction scattering plane. In the latter case, the orientation and alignment of the product angular momentum was found to be significant and strongly dependent on the scattering angle. The calculation also reveals significant differences between the vector correlation properties of the two reactions under study which are due to difference in the reaction mechanisms. In the case of F + HD reaction, the branching ratio between HF and DF production points out interest in the insight gained into the detailed dynamics, when information is available either from exact quantum mechanical calculations or from especially designed experiments. Also, the geometrical arrangement for the experimental determination of the product angular momentum orientation and alignment based on a compact and convenient spherical tensor expression for the intensity of the resonance enhanced multiphoton ionization (REMPI 2 + 1) signal is suggested.

Krasilnikov, Mikhail B.; Popov, Ruslan S.; Roncero, Octavio; De Fazio, Dario; Cavalli, Simonetta; Aquilanti, Vincenzo; Vasyutinskii, Oleg S.

2013-06-01

201

Polarization of molecular angular momentum in the chemical reactions Li + HF and F + HD.  

PubMed

The quantum mechanical approach to vector correlation of angular momentum orientation and alignment in chemical reactions [G. Balint-Kurti and O. S. Vasyutinskii, J. Phys. Chem. A 113, 14281 (2009)] is applied to the molecular reagents and products of the Li + HF [L. Gonzalez-Sanchez, O. S. Vasyutinskii, A. Zanchet, C. Sanz-Sanz, and O. Roncero, Phys. Chem. Chem. Phys. 13, 13656 (2011)] and F + HD [D. De Fazio, J. Lucas, V. Aquilanti, and S. Cavalli, Phys. Chem. Chem. Phys. 13, 8571 (2011)] reactions for which accurate scattering information has become recently available through time-dependent and time-independent approaches. Application of the theory to two important particular cases of the reactive collisions has been considered: (i) the influence of the angular momentum polarization of reactants in the entrance channel on the spatial distribution of the products in the exit channel and (ii) angular momentum polarization of the products of the reaction between unpolarized reactants. In the former case, the role of the angular momentum alignment of the reactants is shown to be large, particularly when the angular momentum is perpendicular to the reaction scattering plane. In the latter case, the orientation and alignment of the product angular momentum was found to be significant and strongly dependent on the scattering angle. The calculation also reveals significant differences between the vector correlation properties of the two reactions under study which are due to difference in the reaction mechanisms. In the case of F + HD reaction, the branching ratio between HF and DF production points out interest in the insight gained into the detailed dynamics, when information is available either from exact quantum mechanical calculations or from especially designed experiments. Also, the geometrical arrangement for the experimental determination of the product angular momentum orientation and alignment based on a compact and convenient spherical tensor expression for the intensity of the resonance enhanced multiphoton ionization (REMPI 2 + 1) signal is suggested. PMID:23822239

Krasilnikov, Mikhail B; Popov, Ruslan S; Roncero, Octavio; De Fazio, Dario; Cavalli, Simonetta; Aquilanti, Vincenzo; Vasyutinskii, Oleg S

2013-06-28

202

Selection of substrata by juvenile Choromytilus chorus (Mytilidae): are chemical cues important?  

Microsoft Academic Search

The importance of filamentous algae, and filamentous structures in general, to patterns of recruitment in mytilids is well documented. Surprisingly, the relative importance of physical and chemical cues in mediating this interaction has rarely been examined for mytilid larvae or plantigrades. Here we examine the distribution and abundance of three mytilid species on three common species of algae on the

Andrew R. Davis; Carlos A. Moreno

1995-01-01

203

SUBSTITUTION REACTIONS FOR THE DETOXIFICATION OF HAZARDOUS CHEMICALS  

EPA Science Inventory

Chemical Treatment is one of several treatment techniques used for the remediation of toxic and hazardous chemicals. Chemical treatment in this report is defined as substitution of halogens by hydrogens for the conversion of halogenated organic toxicant into its native hydrocarb...

204

Theoretical research program to study chemical reactions in AOTV bow shock tubes  

NASA Technical Reports Server (NTRS)

The main focus was the development, implementation, and calibration of methods for performing molecular electronic structure calculations to high accuracy. These various methods were then applied to a number of chemical reactions and species of interest to NASA, notably in the area of combustion chemistry. Among the development work undertaken was a collaborative effort to develop a program to efficiently predict molecular structures and vibrational frequencies using energy derivatives. Another major development effort involved the design of new atomic basis sets for use in chemical studies: these sets were considerably more accurate than those previously in use. Much effort was also devoted to calibrating methods for computing accurate molecular wave functions, including the first reliable calibrations for realistic molecules using full CI results. A wide variety of application calculations were undertaken. One area of interest was the spectroscopy and thermochemistry of small molecules, including establishing small molecule binding energies to an accuracy rivaling, or even on occasion surpassing, the experiment. Such binding energies are essential input to modeling chemical reaction processes, such as combustion. Studies of large molecules and processes important in both hydrogen and hydrocarbon combustion chemistry were also carried out. Finally, some effort was devoted to the structure and spectroscopy of small metal clusters, with applications to materials science problems.

Taylor, Peter R.

1993-01-01

205

Studies in photochemical smog chemistry. 1. Atmospheric chemistry of toulene. 2. Analysis of chemical reaction mechanisms for photochemical smog  

SciTech Connect

This study focuses on two related topics in the gas phase organic chemistry of importance in urban air pollution. An experimental effort aimed at developing a new explicit reaction mechanism for the atmospheric photooxidation of toluene is described. This mechanism is tested using experimental data from both indoor and outdoor smog chamber facilities. The predictions of the new reaction mechanism are found to be in good agreement with both sets of experimental data. Additional simulations performed with the new mechanism are used to investigate various mechanistic paths. A theoretical analysis of lumped chemical reaction mechanisms for photochemical smog is presented. Included is a description of a new counter species analysis technique which can be used to analyze any complex chemical reaction mechanism. When applied to mechanisms for photochemical smog, this analysis is shown capable of providing answers to previously inaccessible questions such as the relative contributions of individual organics to photochemical ozone formation.

Leone, J.A.

1985-01-01

206

Separation of chemical reaction intermediates by metal-organic frameworks.  

PubMed

HPLC columns custom-packed with metal-organic framework (MOF) materials are used for the separation of four small intermediates and byproducts found in the commercial synthesis of an important active pharmaceutical ingredient in methanol. In particular, two closely related amines can be separated in the methanol reaction medium using MOFs, but not with traditional C18 columns using an optimized aqueous mobile phase. Infrared spectroscopy, UV-vis spectroscopy, X-ray diffraction, and thermogravimetric analysis are used in combination with molecular dynamic simulations to study the separation mechanism for the best-performing MOF materials. It is found that separation with ZIF-8 is the result of an interplay between the thermodynamic driving force for solute adsorption within the framework pores and the kinetics of solute diffusion into the material pores, while the separation with Basolite F300 is achieved because of the specific interactions between the solutes and Fe(3+) sites. This work, and the exceptional ability to tailor the porous properties of MOF materials, points to prospects for using MOF materials for the continuous separation and synthesis of pharmaceutical compounds. PMID:21626684

Centrone, Andrea; Santiso, Erik E; Hatton, T Alan

2011-08-22

207

Chemical Diffusivity and Wave Propagation in Surface Reactions: Analysis of the Bistable Monomer-Dimer Reaction Model  

NASA Astrophysics Data System (ADS)

Spatial pattern formation and wave propagation in surface reactions is controlled in part by chemical diffusion of the adsorbed reactant species. While in the simplest treatment, the chemical diffusion coefficients are taken as constant, the diffusion of one species is in fact influenced by the presence of coadsorbed species(M. Tammaro, M. Sabella, and J.W. Evans, J. Chem. Phys. 103 (1995) 10277; M. Tammaro and J.W. Evans, J. Chem. Phys. (submitted)). We thus develop an appropriate theory for chemical diffusion in mixed adlayers of mobile species. This theory is used to analyze the propagation of chemical waves associated with the displacement of the CO-poisoned state by a reactive steady state in the monomer-dimer model for CO-oxidation. We find good agreement between "exact" simulation results and those obtained from reaction-diffusion equations incorporating an appropriate description of the non-linear chemical diffusion.

Tammaro, M.; Evans, J. W.

1997-03-01

208

Using Drawing Technology to Assess Students' Visualizations of Chemical Reaction Processes  

NASA Astrophysics Data System (ADS)

In this study, we investigated how students used a drawing tool to visualize their ideas of chemical reaction processes. We interviewed 30 students using thinking-aloud and retrospective methods and provided them with a drawing tool. We identified four types of connections the students made as they used the tool: drawing on existing knowledge, incorporating dynamic aspects of chemical processes, linking a visualization to the associated chemical phenomenon, and connecting between the visualization and chemistry concepts. We also compared students who were able to create dynamic visualizations with those who only created static visualizations. The results indicated a relationship between students constructing a dynamic view of chemical reaction processes and their understanding of chemical reactions. This study provides insights into the use of visualizations to support instruction and assessment to facilitate students' integrated understanding of chemical reactions.

Chang, Hsin-Yi; Quintana, Chris; Krajcik, Joseph

2014-06-01

209

Iteration Scheme for Implicit Calculations of Kinetic and Equilibrium Chemical Reactions in Fluid Dynamics  

NASA Astrophysics Data System (ADS)

An iteration scheme for the implicit treatment of equilibrium chemical reactions in partial equilibrium flow has previously been described (J. D. Ramshaw and A. A. Amsden, J. Comput. Phys.59, 484 (1985); 71 , 224 (1987)). Here we generalize this scheme to kinetic reactions as well as equilibrium reactions. This extends the applicability of the scheme to problems with kinetic reactions that are fast in some regions of the flow field but slow in others. The resulting scheme thereby provides a single unified framework for the implicit treatment of an arbitrary number of coupled equilibrium and kinetic reactions in chemically reacting fluid flow.

Ramshaw, J. D.; Chang, C. H.

1995-02-01

210

Iteration scheme for implicit calculations of kinetic and equilibrium chemical reactions in fluid dynamics  

SciTech Connect

An iteration scheme for the implicit treatment of equilibrium chemical reactions in partial equilibrium flow has previously been described. Here we generalize this scheme to kinetic reactions as well as equilibrium reactions. This extends the applicability of the scheme to problems with kinetic reactions that are fast in regions of the flow field but slow in others. The resulting scheme thereby provides a single unified framework for the implicit treatment of an arbitrary number of coupled equilibrium and kinetic reactions in chemically reacting fluid flow. 10 refs., 2 figs.

Ramshaw, J.D.; Chang, C.H. [Idaho National Engineering Lab., ID (United States)] [Idaho National Engineering Lab., ID (United States)

1995-02-01

211

Direct observation of chemical reactions on single gold nanocrystals using surface plasmon spectroscopy  

Microsoft Academic Search

Heterogeneous catalysts have been pivotal to the development of the modern chemical industry and are essential for catalysing many industrial reactions. However, reaction rates are different for every individual catalyst particle and depend upon each particle's morphology and size, crystal structure and composition. Measuring the rates of reaction on single nanocrystals will enable the role of catalyst structure to be

Carolina Novo; Alison M. Funston; Paul Mulvaney

2008-01-01

212

A Unified Approach to the Study of Chemical Reactions in Freshman Chemistry.  

ERIC Educational Resources Information Center

Provides rationale and objectives for presenting chemical reactions in a unified, logical six-stage approach rather than a piecemeal approach. Stages discussed include: introduction, stable electronic configurations and stable oxidation states, reactions between two free elements, ion transfer/proton transfer reactions, double displacement…

Cassen, T.; DuBois, Thomas D.

1982-01-01

213

Variational principles for describing chemical reactions: Condensed reactivity indices  

NASA Astrophysics Data System (ADS)

Two recent papers [P. W. Ayers and R. G. Parr, J. Am. Chem. Soc. 122, 2010 (2000); 123, 2007 (2001)] have shown how variational principles for the energy may be used to derive and elucidate the significance of the chemical reactivity indices of density-functional theory. Here, similar ideas are applied, yielding a systematic, mathematically rigorous, and physically sound approach to condensed reactivity indices. First, we use the variational principle for the energy to derive an expression for the condensed Fukui function index in terms of the condensed hardness kernel. Next, we address an important open problem pertaining to condensed reactivity indices: when (if ever) is the condensed Fukui function for an atom in a molecule negative? In particular, our analysis confirms the observation, hitherto based only on computational evidence, that the Hirshfeld partitioning is optimal for obtaining non-negative Fukui functions. We also hypothesize that the strong diagonal dominance of the condensed hardness kernel is sufficient for the non-negativity of the Fukui function. Errors in the partitioning of molecules into atoms and inadequate treatment of correlation are pinpointed as the most likely causes of negative condensed Fukui functions. We conclude by noting that the condensed Fukui functions are, in some respects, more appropriate indicators of a molecular site's reactivity than the Fukui function itself.

Ayers, Paul W.; Morrison, Robert C.; Roy, Ram K.

2002-05-01

214

Study of chemical reactions under the influence of ultrasound  

NASA Astrophysics Data System (ADS)

At Los Alamos, the author is interested in sonochemistry because there is potential for accelerating reactions involving the synthesis of certain nitro compounds and for reducing the possibility of decomposition under milder reaction conditions. The author has initiated the study of the nitration of 2,4-dihydro-3H-1,2,4-triazol-3-one with concentrated nitric acid under sonication. The preparation of 4,5-tetrazine, and oxidation of 3,6-diamino-1,2,4,5-tetrazine were also studied. Sonication reaction conditions and results of these reactions under ultrasound are discussed in detail.

Lee, Kien-Yin

215

Chemical mechanism and kinetics study on the ocimene ozonolysis reaction in atmosphere  

NASA Astrophysics Data System (ADS)

The ocimene ozonolysis reaction is one of the most important processes for the formation of secondary organic aerosol (SOA). In this paper, molecular orbital theory has been performed for the reaction of ocimene with O 3, and the detailed reaction mechanisms of active intermediates with H 2O or NO are also presented. The geometry parameters and vibrational frequencies of the stationary points are calculated at the MPWB1K level with the 6-31G(d,p) basis set. Single-point energy calculations are carried out at the MPWB1K/6-311+G(3df,2p) level. On the basis of the quantum chemical information, the Rice-Ramsperger-Kassel-Marcus (RRKM) theory and the canonical variational transition state theory (CVT) with small-curvature tunneling effect (SCT) are used to calculate the rate constants over the temperature range of 200-800 K. The arrhenius formulas of rate constants with the temperature are fitted, which can provide helpful information for the model simulation study. The atmospheric lifetimes of the reaction species are estimated according to the rate constants.

Sun, Xiaomin; Bai, Jing; Zhao, Yuyang; Zhang, Chenxi; Wang, Yudong; Hu, Jingtian

2011-11-01

216

Vicher: A Virtual Reality Based Educational Module for Chemical Reaction Engineering.  

ERIC Educational Resources Information Center

A virtual reality application for undergraduate chemical kinetics and reactor design education, Vicher (Virtual Chemical Reaction Model) was originally designed to simulate a portion of a modern chemical plant. Vicher now consists of two programs: Vicher I that models catalyst deactivation and Vicher II that models nonisothermal effects in…

Bell, John T.; Fogler, H. Scott

1996-01-01

217

Kinetics of thermochemical gas-solid reactions important in the Venus sulfur cycle  

NASA Technical Reports Server (NTRS)

The thermochemical net reaction CaCO3 + SO2 yields CaSO4 + CO is predicted to be an important sink for incorporation of SO2 into the Venus crust. The reaction rate law was established to understand the dependence of rate on experimental variables such as temperature and partial pressure of SO2, CO2, and O2. The experimental approach was a variant of the thermogravimetric method often employed to study the kinetics of thermochemical gas-solid reactions. Clear calcite crystals were heated at constant temperature in SO2-bearing gas streams for varying time periods. Reaction rate was determined by three independent methods. A weighted linear least squares fit to all rate data yielded a rate equation. Based on the Venera 13, 14 and Vega 2 observations of CaO content of the Venus atmosphere, SO2 at the calculated rate would be removed from the Venus atmosphere in about 1,900,00 years. The most plausible endogenic source of the sulfur needed to replenish atmospheric SO2 is volcanism. The annual amount of erupted material needed for the replenishment depends on sulfur content; three ratios are used to calculate rates ranging from 0.4 to 11 cu km/year. This geochemically derived volcanism rate can be used to test if geophysically derived rates are correct. The work also suggests that Venus is less volcanically active than the Earth.

Fegley, Bruce, Jr.

1988-01-01

218

Kinetics of thermochemical gas-solid reactions important in the Venus sulfur cycle  

NASA Astrophysics Data System (ADS)

The thermochemical net reaction CaCO3 + SO2 yields CaSO4 + CO is predicted to be an important sink for incorporation of SO2 into the Venus crust. The reaction rate law was established to understand the dependence of rate on experimental variables such as temperature and partial pressure of SO2, CO2, and O2. The experimental approach was a variant of the thermogravimetric method often employed to study the kinetics of thermochemical gas-solid reactions. Clear calcite crystals were heated at constant temperature in SO2-bearing gas streams for varying time periods. Reaction rate was determined by three independent methods. A weighted linear least squares fit to all rate data yielded a rate equation. Based on the Venera 13, 14 and Vega 2 observations of CaO content of the Venus atmosphere, SO2 at the calculated rate would be removed from the Venus atmosphere in about 1,900,00 years. The most plausible endogenic source of the sulfur needed to replenish atmospheric SO2 is volcanism. The annual amount of erupted material needed for the replenishment depends on sulfur content; three ratios are used to calculate rates ranging from 0.4 to 11 cu km/year. This geochemically derived volcanism rate can be used to test if geophysically derived rates are correct. The work also suggests that Venus is less volcanically active than the Earth.

Fegley, Bruce, Jr.

1988-07-01

219

Chemical reactions on metal oxide surfaces investigated by vibrational spectroscopy  

Microsoft Academic Search

The most successful method to unravel the microscopic mechanisms governing reactions in heterogeneous catalysis is the “surface science” approach which is based on well-controlled studies on model catalysts (usually single crystal surfaces) under ultrahigh vacuum (UHV) conditions [G. Ertl, Angew. Chem. 47 (2008) 3524]. In this review our recent vibrational spectroscopic studies on selected model reactions at various single-crystalline metal

Yuemin Wang; Christof Wöll

2009-01-01

220

A kinetic mechanism inducing oscillations in simple chemical reactions networks.  

PubMed

It is known that a kinetic reaction network in which one or more secondary substrates are acting as cofactors may exhibit an oscillatory behavior. The aim of this work is to provide a description of the functional form of such a cofactor action guaranteeing the onset of oscillations in sufficiently simple reaction networks. PMID:20462291

Coatleven, Julien; Altafini, Claudio

2010-04-01

221

CHEMICAL REACTIONS OF AQUATIC HUMIC MATERIALS WITH SELECTED OXIDANTS  

EPA Science Inventory

A study was conducted to identify the specific organic reaction products of natural aquatic humic materials with selected oxidants (KMnO4, HOCl, Cl02, O3 and monochloramine). Reaction products were identified by GC/MS after solvent extraction and derivatization. The two most reac...

222

Chemical Reactions of Aquatic Humic Materials with Selected Oxidants.  

National Technical Information Service (NTIS)

A study was conducted to identify the specific organic reaction products of natural aquatic humic materials with selected oxidants (KMnO4, HOCl, Cl02, O3 and monochloramine). Reaction products were identified by GC/MS after solvent extraction and derivati...

A. A. Stevens D. S. Millington J. D. Johnson R. F. Christman

1983-01-01

223

Theoretical study of thermodynamic properties and reaction rates of importance in the high-speed research program  

NASA Technical Reports Server (NTRS)

One of the primary goals of NASA's high-speed research program is to determine the feasibility of designing an environmentally safe commercial supersonic transport airplane. The largest environmental concern is focused on the amount of ozone destroying nitrogen oxides (NO(x)) that would be injected into the lower stratosphere during the cruise portion of the flight. The limitations placed on NO(x) emission require more than an order of magnitude reduction over current engine designs. To develop strategies to meet this goal requires first gaining a fundamental understanding of the combustion chemistry. To accurately model the combustor requires a computational fluid dynamics approach that includes both turbulence and chemistry. Since many of the important chemical processes in this regime involve highly reactive radicals, an experimental determination of the required thermodynamic data and rate constants is often very difficult. Unlike experimental approaches, theoretical methods are as applicable to highly reactive species as stable ones. Also our approximation of treating the dynamics classically becomes more accurate with increasing temperature. In this article we review recent progress in generating thermodynamic properties and rate constants that are required to understand NO(x) formation in the combustion process. We also describe our one-dimensional modeling efforts to validate an NH3 combustion reaction mechanism. We have been working in collaboration with researchers at LeRC, to ensure that our theoretical work is focused on the most important thermodynamic quantities and rate constants required in the chemical data base.

Langhoff, Stephen; Bauschlicher, Charles; Jaffe, Richard

1992-01-01

224

Molecule-based approach for computing chemical-reaction rates in upper atmosphere hypersonic flows.  

SciTech Connect

This report summarizes the work completed during FY2009 for the LDRD project 09-1332 'Molecule-Based Approach for Computing Chemical-Reaction Rates in Upper-Atmosphere Hypersonic Flows'. The goal of this project was to apply a recently proposed approach for the Direct Simulation Monte Carlo (DSMC) method to calculate chemical-reaction rates for high-temperature atmospheric species. The new DSMC model reproduces measured equilibrium reaction rates without using any macroscopic reaction-rate information. Since it uses only molecular properties, the new model is inherently able to predict reaction rates for arbitrary nonequilibrium conditions. DSMC non-equilibrium reaction rates are compared to Park's phenomenological non-equilibrium reaction-rate model, the predominant model for hypersonic-flow-field calculations. For near-equilibrium conditions, Park's model is in good agreement with the DSMC-calculated reaction rates. For far-from-equilibrium conditions, corresponding to a typical shock layer, the difference between the two models can exceed 10 orders of magnitude. The DSMC predictions are also found to be in very good agreement with measured and calculated non-equilibrium reaction rates. Extensions of the model to reactions typically found in combustion flows and ionizing reactions are also found to be in very good agreement with available measurements, offering strong evidence that this is a viable and reliable technique to predict chemical reaction rates.

Gallis, Michail A.; Bond, Ryan Bomar; Torczynski, John Robert

2009-08-01

225

Study of chemical reactions under the influence of ultrasound  

SciTech Connect

At Los Alamos the author is interested in sonochemistry because there is potential for accelerating reactions involving the synthesis of certain nitro compounds and for reducing the possibility of decomposition under milder reaction conditions. The author has initiated the study of the nitration of 2,4-dihydro-3H-1,2,4-triazol-3-one with concentrated nitric acid under sonication. The preparation of 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2-dihydro-1,2,4,5-tetrazine, and oxidation of 3,6-diamino-1,2,4,5-tetrazine were also studied. Sonication reaction conditions and results of these reactions under ultrasound are discussed in detail.

Lee, Kien-Yin

1993-07-01

226

Study of chemical reactions under the influence of ultrasound  

SciTech Connect

At Los Alamos the author is interested in sonochemistry because there is potential for accelerating reactions involving the synthesis of certain nitro compounds and for reducing the possibility of decomposition under milder reaction conditions. The author has initiated the study of the nitration of 2,4-dihydro-3H-1,2,4-triazol-3-one with concentrated nitric acid under sonication. The preparation of 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2-dihydro-1,2,4,5-tetrazine, and oxidation of 3,6-diamino-1,2,4,5-tetrazine were also studied. Sonication reaction conditions and results of these reactions under ultrasound are discussed in detail.

Lee, Kien-Yin.

1993-01-01

227

Lagrangian simulation of mixing-controlled chemical reactions  

NASA Astrophysics Data System (ADS)

A Lagrangian framework is developed to simulate the mixing-limited bimolecular second-order reactions in laboratory-scale porous media. The core is to define the interaction radius of reactant molecule, which controls the probability of reactions and is affected by the property of the medium, reactants and fluids. The simulated particle dynamics is checked against the kinetics for both diffusion-controlled and well-mixed reactions. The influence of anomalous transport on mixing and hence reaction is also evaluated, where both the space and time fractional-derivative models are considered. Applicability of the simulator is further tested by simulating the breakthrough curves and snapshots for different products observed in previous laboratory experiments.

Zhang, Y.

2010-12-01

228

Chemoselectivity in Chemical Biology: Acyl Transfer Reactions with Sulfur and Selenium  

PubMed Central

CONSPECTUS A critical source of insight into biological function is derived from the chemist’s ability to create new covalent bonds between molecules, whether they are endogenous or exogenous to a biological system. A daunting impediment to selective bond formation, however, is the myriad of reactive functionalities present in biological milieu. The high reactivity of the most abundant molecule in biology—water—makes the challenges all the more difficult. We have met these challenges by exploiting the reactivity of sulfur and selenium in acyl transfer reactions. The reactivity of both sulfur and selenium is high compared to that of their chalcogen congener, oxygen. In this Account, we highlight recent developments in this arena, emphasizing contributions from our laboratory. One focus of our research is furthering the chemistry of native chemical ligation (NCL) and expressed protein ligation (EPL), two related processes that enable the synthesis and semisynthesis of proteins. These techniques exploit the lower pKa of thiols and selenols relative to alcohols. Although a deprotonated hydroxyl group in the side chain of a serine residue is exceedingly rare in a biological context, the pKa value of the thiol in cysteine (8.5) and of the selenol in selenocysteine (5.7) often render these side chains anionic under physiological conditions. NCL and EPL take advantage of the high nucleophilicity of the thiolate as well as its utility as a leaving group, and we have expanded the scope of these methods to include selenocysteine. Although the genetic code limits the components of natural proteins to 20 or so ?-amino acids, NCL and EPL enable the semisynthetic incorporation of a limitless variety of nonnatural modules into proteins. These modules are enabling chemical biologists to interrogate protein structure and function with unprecedented precision. We are also pursuing the further development of the traceless Staudinger ligation, through which a phosphinothioester and azide form an amide. We first reported this chemical ligation method, which leaves no residual atoms in the product, in 2000. Our progress in effecting the reaction in water, without an organic co-solvent, was an important step in the expansion of its utility. Moreover, we have developed the traceless Staudinger reaction as a means for immobilizing proteins on a solid support, providing a general method of fabricating microarrays that display proteins in a uniform orientation. Along with NCL and EPL, the traceless Staudinger ligation has made proteins more readily accessible targets for chemical synthesis and semisynthesis. The underlying acyl transfer reactions with sulfur and selenium provide an efficient means to synthesize, remodel, and immobilize proteins, and they have enabled us to interrogate biological systems.

McGrath, Nicholas A.; Raines, Ronald T.

2011-01-01

229

Chemoselectivity in chemical biology: acyl transfer reactions with sulfur and selenium.  

PubMed

A critical source of insight into biological function is derived from the chemist's ability to create new covalent bonds between molecules, whether they are endogenous or exogenous to a biological system. A daunting impediment to selective bond formation, however, is the myriad of reactive functionalities present in biological milieu. The high reactivity of the most abundant molecule in biology, water, makes the challenges all the more difficult. We have met these challenges by exploiting the reactivity of sulfur and selenium in acyl transfer reactions. The reactivity of both sulfur and selenium is high compared with that of their chalcogen congener, oxygen. In this Account, we highlight recent developments in this arena, emphasizing contributions from our laboratory. One focus of our research is furthering the chemistry of native chemical ligation (NCL) and expressed protein ligation (EPL), two related processes that enable the synthesis and semisynthesis of proteins. These techniques exploit the lower pK(a) of thiols and selenols relative to alcohols. Although a deprotonated hydroxyl group in the side chain of a serine residue is exceedingly rare in a biological context, the pK(a) values of the thiol in cysteine (8.5) and of the selenol in selenocysteine (5.7) often render these side chains anionic under physiological conditions. NCL and EPL take advantage of the high nucleophilicity of the thiolate as well as its utility as a leaving group, and we have expanded the scope of these methods to include selenocysteine. Although the genetic code limits the components of natural proteins to 20 or so ?-amino acids, NCL and EPL enable the semisynthetic incorporation of a limitless variety of nonnatural modules into proteins. These modules are enabling chemical biologists to interrogate protein structure and function with unprecedented precision. We are also pursuing the further development of the traceless Staudinger ligation, through which a phosphinothioester and azide form an amide. We first reported this chemical ligation method, which leaves no residual atoms in the product, in 2000. Our progress in effecting the reaction in water, without an organic cosolvent, was an important step in the expansion of its utility. Moreover, we have developed the traceless Staudinger reaction as a means for immobilizing proteins on a solid support, providing a general method of fabricating microarrays that display proteins in a uniform orientation. Along with NCL and EPL, the traceless Staudinger ligation has made proteins more readily accessible targets for chemical synthesis and semisynthesis. The underlying acyl transfer reactions with sulfur and selenium provide an efficient means to synthesize, remodel, and immobilize proteins, and they have enabled us to interrogate biological systems. PMID:21639109

McGrath, Nicholas A; Raines, Ronald T

2011-09-20

230

A Petri net approach to the study of persistence in chemical reaction networks.  

PubMed

Persistence is the property, for differential equations in R(n), that solutions starting in the positive orthant do not approach the boundary of the orthant. For chemical reactions and population models, this translates into the non-extinction property: provided that every species is present at the start of the reaction, no species will tend to be eliminated in the course of the reaction. This paper provides checkable conditions for persistence of chemical species in reaction networks, using concepts and tools from Petri net theory, and verifies these conditions on various systems which arise in the modeling of cell signaling pathways. PMID:17869313

Angeli, David; De Leenheer, Patrick; Sontag, Eduardo D

2007-12-01

231

A human-specific mitochondrial antibody its importance in the identification of organ-specific reactions.  

PubMed

A previously unrecognized autoantibody, detected by immunofluorescence, reacted with all human organs but gave negative results on tissues from rat, mouse, rabbit, guinea-pig, calf and chicken. From its predilection for mitochondria-rich cells (oncocytes) and its selective absorption with human but not animal mitochondria, it was identified as an anti-human mitochondrial antibody and named AHMA. The antibody is found in about 1% of normal subjects and is mostly of IgG class and of low titres. Its prevalence is increased in primary biliary cirrhosis where it may be associated with the standard non-species-specific AMA used for the differential diagnosis of this disease. The importance of AHMA is mainly in possible confusion with organ-specific reactions in submaxillary duct, parathyroid oxyphil cells and in trying to identify new endocrine cells such as those producing pancreatic polypeptide (HPP) in human tissues. Animals immunized with human hormones develop reactions to human mitochondria and thus produce misleading immunofluorescence reactions when used in low dilutions. PMID:330060

Swana, G T; Swana, M R; Bottazzo, G F; Doniach, D

1977-06-01

232

A human-specific mitochondrial antibody its importance in the identification of organ-specific reactions.  

PubMed Central

A previously unrecognized autoantibody, detected by immunofluorescence, reacted with all human organs but gave negative results on tissues from rat, mouse, rabbit, guinea-pig, calf and chicken. From its predilection for mitochondria-rich cells (oncocytes) and its selective absorption with human but not animal mitochondria, it was identified as an anti-human mitochondrial antibody and named AHMA. The antibody is found in about 1% of normal subjects and is mostly of IgG class and of low titres. Its prevalence is increased in primary biliary cirrhosis where it may be associated with the standard non-species-specific AMA used for the differential diagnosis of this disease. The importance of AHMA is mainly in possible confusion with organ-specific reactions in submaxillary duct, parathyroid oxyphil cells and in trying to identify new endocrine cells such as those producing pancreatic polypeptide (HPP) in human tissues. Animals immunized with human hormones develop reactions to human mitochondria and thus produce misleading immunofluorescence reactions when used in low dilutions.

Swana, G T; Swana, M R; Bottazzo, G F; Doniach, D

1977-01-01

233

Characterisation of hydrocarbonaceous overlayers important in metal-catalysed selective hydrogenation reactions  

NASA Astrophysics Data System (ADS)

The hydrogenation of alkynes to alkenes over supported metal catalysts is an important industrial process and it has been shown that hydrocarbonaceous overlayers are important in controlling selectivity profiles of metal-catalysed hydrogenation reactions. As a model system, we have selected propyne hydrogenation over a commercial Pd(5%)/Al2O3 catalyst. Inelastic neutron scattering studies show that the C–H stretching mode ranges from 2850 to 3063 cm?1, indicating the mostly aliphatic nature of the overlayer and this is supported by the quantification of the carbon and hydrogen on the surface. There is also a population of strongly hydrogen-bonded hydroxyls, their presence would indicate that the overlayer probably contains some oxygen functionality. There is little evidence for any olefinic or aromatic species. This is distinctly different from the hydrogen-poor overlayers that are deposited on Ni/Al2O3 catalysts during methane reforming.

Lennon, David; Warringham, Robbie; Guidi, Tatiana; Parker, Stewart F.

2013-12-01

234

Role of Conformational Structures and Torsional Anharmonicity in Controlling Chemical Reaction Rates and Relative Yields: Butanal + HO2 Reactions  

SciTech Connect

Aldehyde–radical reactions are important in atmospheric and combustion chemistry, and the reactions studied here also serve more generally to illustrate a fundamental aspect of chemical kinetics that has been relatively unexplored from a quantitative point of view, in particular the roles of multiple structures and torsional anharmonicity in determining the rate constants and branching ratios (product yields). We consider hydrogen abstraction from four carbon sites of butanal (carbonyl-C, a-C, b-C and g-C) by hydroperoxyl radical. We employed multi-structural variational transition state theory for studying the first three channels; this uses a multi-faceted dividing surface and allows us to include the contributions of multiple structures of both reacting species and transition states. Multiconfigurational Shepard interpolation (MCSI) was used to obtain the geometries and energies of the potential energy surface along the minimum-energy paths, with gradients and Hessians calculated by the M08-HX/maug-cc-pVTZ method. We find the numbers of structures obtained for the transition states are 46, 60, 72 and 76respectively for the H abstraction at the carbonyl C, the a position, the b position and the g position. Our results show that neglecting the factors arising from multiple structures and torsional anharmonicity would lead to errors at 300, 1000 and 2400 K of factors of 8, 11 and 10 for abstraction at the carbonyl-O, 2, 11 and 25 at the a-C position, 2, 23 and 47 at the b-C position, and 0.6, 8 and 18 at the g-C position. The errors would be even larger at high temperature for the reverse of the H abstraction at the b-C. Relative yields are changed as much as a factor of 7.0 at 200 K, a factor of 5.0 at 298 K, and a factor of 3.7 in the other direction at 2400 K. The strong dependence of the product ratios on the multi-structural anharmonicity factors shows that such factors play an important role in controlling branching ratios in reaction mechanism networks.

Zheng, Jingjing; Seal, Prasenjit; Truhlar, Donald G.

2012-09-24

235

Ab initio chemical kinetics for the HCCO + OH reaction  

NASA Astrophysics Data System (ADS)

The mechanism for the reaction of HCCO and OH has been investigated at different high-levels of theory. The reaction was found to occur on singlet and triplet potential energy surfaces with multiple accessible paths. Rate constants predicted by variational RRKM/ME calculations show that the reaction on both surfaces occurs primarily by barrierless OH attack at both C atoms producing excited intermediates which fragment to produce predominantly CO and 1,3HCOH with kS = 3.12 × 10-8T-0.59exp[-73.0/T] and kT = 6.29 × 10-11T0.13exp[108/T] cm3 molecule-1 s-1 at T = 300-2000 K, independent of pressure at P < 76 000 Torr.

Mai, Tam V.-T.; Raghunath, P.; Le, Xuan T.; Huynh, Lam K.; Nam, Pham-Cam; Lin, M. C.

2014-01-01

236

Energy storage and transport by reversible chemical reactions  

NASA Astrophysics Data System (ADS)

Reversible thermochemical reactions are one of the possibilities to store and transport high temperature heat (800 K up to 1300 K). There are open cycles and closed cycles. Some reversible systems are described, as the SO2-SO3 system. A typical example of open cycle is the reaction for water decomposition. Results of a pilot plant to verify the decomposition of sulfuric acid are described; the technological feasibility of this method for hydrogen production is shown. The possibility to use other high temperature sources, as solar energy is discussed.

Beghi, G.

237

Chemical reaction and equilibration mechanisms in detonation waves  

SciTech Connect

Experimental and theoretical evidence for the nonequilibrium Zeldovich-von Neumann-Doring (NEZND) theory of self-sustaining detonation is presented. High density, high temperature transition state theory is used to calculate unimolecular reaction rate constants for the initial decomposition of gaseous norbornene, liquid nitromethane, and solid, single crystal pentaerythritol tetranitrate as functions of shock temperature. The calculated rate constants are compared to those derived from experimental induction time measurements at various shock and detonation states. Uncertainties in the calculated shock and von Neumann spike temperatures are the main drawbacks to calculating these reaction rates. Nanosecond measurements of the shock temperatures of unreacted explosives are necessary to reduce these uncertainties.

Tarver, C. M., LLNL

1997-07-01

238

Large-scale prediction of adverse drug reactions using chemical, biological, and phenotypic properties of drugs  

PubMed Central

Objective Adverse drug reaction (ADR) is one of the major causes of failure in drug development. Severe ADRs that go undetected until the post-marketing phase of a drug often lead to patient morbidity. Accurate prediction of potential ADRs is required in the entire life cycle of a drug, including early stages of drug design, different phases of clinical trials, and post-marketing surveillance. Methods Many studies have utilized either chemical structures or molecular pathways of the drugs to predict ADRs. Here, the authors propose a machine-learning-based approach for ADR prediction by integrating the phenotypic characteristics of a drug, including indications and other known ADRs, with the drug's chemical structures and biological properties, including protein targets and pathway information. A large-scale study was conducted to predict 1385 known ADRs of 832 approved drugs, and five machine-learning algorithms for this task were compared. Results This evaluation, based on a fivefold cross-validation, showed that the support vector machine algorithm outperformed the others. Of the three types of information, phenotypic data were the most informative for ADR prediction. When biological and phenotypic features were added to the baseline chemical information, the ADR prediction model achieved significant improvements in area under the curve (from 0.9054 to 0.9524), precision (from 43.37% to 66.17%), and recall (from 49.25% to 63.06%). Most importantly, the proposed model successfully predicted the ADRs associated with withdrawal of rofecoxib and cerivastatin. Conclusion The results suggest that phenotypic information on drugs is valuable for ADR prediction. Moreover, they demonstrate that different models that combine chemical, biological, or phenotypic information can be built from approved drugs, and they have the potential to detect clinically important ADRs in both preclinical and post-marketing phases.

Liu, Mei; Wu, Yonghui; Chen, Yukun; Sun, Jingchun; Zhao, Zhongming; Chen, Xue-wen; Matheny, Michael Edwin

2012-01-01

239

LSENS, a general chemical kinetics and sensitivity analysis code for gas-phase reactions: User's guide  

NASA Technical Reports Server (NTRS)

A general chemical kinetics and sensitivity analysis code for complex, homogeneous, gas-phase reactions is described. The main features of the code, LSENS, are its flexibility, efficiency and convenience in treating many different chemical reaction models. The models include static system, steady, one-dimensional, inviscid flow, shock initiated reaction, and a perfectly stirred reactor. In addition, equilibrium computations can be performed for several assigned states. An implicit numerical integration method, which works efficiently for the extremes of very fast and very slow reaction, is used for solving the 'stiff' differential equation systems that arise in chemical kinetics. For static reactions, sensitivity coefficients of all dependent variables and their temporal derivatives with respect to the initial values of dependent variables and/or the rate coefficient parameters can be computed. This paper presents descriptions of the code and its usage, and includes several illustrative example problems.

Radhakrishnan, Krishnan; Bittker, David A.

1993-01-01

240

A new approximate method for the stochastic simulation of chemical systems: the representative reaction approach.  

PubMed

We have developed two new approximate methods for stochastically simulating chemical systems. The methods are based on the idea of representing all the reactions in the chemical system by a single reaction, i.e., by the "representative reaction approach" (RRA). Discussed in the article are the concepts underlying the new methods along with flowchart with all the steps required for their implementation. It is shown that the two RRA methods {with the reaction 2A ?> B as the representative reaction (RR)} perform creditably with regard to accuracy and computational efficiency, in comparison to the exact stochastic simulation algorithm (SSA) developed by Gillespie and are able to successfully reproduce at least the first two moments of the probability distribution of each species in the systems studied. As such, the RRA methods represent a promising new approach for stochastically simulating chemical systems. PMID:22108838

Kadam, Shantanu; Vanka, Kumar

2012-01-30

241

Theoretical Studies of Chemical Reactions following Electronic Excitation  

NASA Technical Reports Server (NTRS)

The use of multi-configurational wave functions is demonstrated for several processes: tautomerization reactions in the ground and excited states of the DNA base adenine, dissociation of glycine molecule after electronic excitation, and decomposition/deformation of novel rare gas molecules HRgF. These processes involve bond brealung/formation and require multi-configurational approaches that include dynamic correlation.

Chaban, Galina M.

2003-01-01

242

Enhanced Oxidation and Solvolysis Reactions in Chemically Inert Microheterogeneous Systems.  

National Technical Information Service (NTIS)

In the second year of contract, the following parts of the original project have been accomplished: 1. Model calculations of the kinetics of the decomposition of H2O2 in H2O and fitting of the model of sequence and rates of reactions to the experimental r...

A. M. Braun

1987-01-01

243

Theoretical Chemical Dynamics Studies of Elementary Combustion Reactions.  

National Technical Information Service (NTIS)

The objective of this research was to develop and apply methods for more accurate predictions of reaction rates based on high-level quantum chemistry. We have developed and applied efficient, robust methods for fitting global ab initio potential energy su...

D. L. Thompson

2009-01-01

244

Chemical Principles Revisited. Redox Reactions and the Electropotential Axis.  

ERIC Educational Resources Information Center

This paper suggests a nontraditional pedagogic approach to the subject of redox reactions and electrode potentials suitable for freshman chemistry. Presented is a method for the representation of galvanic cells without the introduction of the symbology and notation of conventional cell diagrams. (CW)

Vella, Alfred J.

1990-01-01

245

Transient modeling of chemical vapor infiltration of methane using multi-step reaction and deposition models  

Microsoft Academic Search

Based on multi-step reaction and deposition models including the hydrogen inhibition model of pyrocarbon growth, transient 2D simulations of chemical vapor infiltration of methane were carried out by a finite element method (FEM) coupling the mass transfer (by convection and diffusion) and the evolutive surface area model with gas-phase and surface chemical reactions. The continuous infiltration, pyrolysis and deposition of

Aijun Li; Olaf Deutschmann

2007-01-01

246

ABIOTIC REDUCTION REACTIONS OF ANTHROPOGENIC ORGANIC CHEMICALS IN ANAEROBIC SYSTEMS: A CRITICAL REVIEW  

EPA Science Inventory

The review is predicated upon the need for a detailed process-level understanding of factors influencing the reduction of anthropogenic organic chemicals in natural waters. In particular, abiotic reductions of anthropogenic organic chemicals are reviewed. The most important reduc...

247

Analytical Solution of Steady State Equations for Chemical Reaction Networks with Bilinear Rate Laws  

PubMed Central

True steady states are a rare occurrence in living organisms, yet their knowledge is essential for quasi-steady state approximations, multistability analysis, and other important tools in the investigation of chemical reaction networks (CRN) used to describe molecular processes on the cellular level. Here we present an approach that can provide closed form steady-state solutions to complex systems, resulting from CRN with binary reactions and mass-action rate laws. We map the nonlinear algebraic problem of finding steady states onto a linear problem in a higher dimensional space. We show that the linearized version of the steady state equations obeys the linear conservation laws of the original CRN. We identify two classes of problems for which complete, minimally parameterized solutions may be obtained using only the machinery of linear systems and a judicious choice of the variables used as free parameters. We exemplify our method, providing explicit formulae, on CRN describing signal initiation of two important types of RTK receptor-ligand systems, VEGF and EGF-ErbB1.

Halasz, Adam M.; Lai, Hong-Jian; McCabe, Meghan M.; Radhakrishnan, Krishnan; Edwards, Jeremy S.

2014-01-01

248

Chemical reactions in protoplanetary accretion disks. IV. Multicomponent dust mixture  

Microsoft Academic Search

We consider the different major components of the dust mixture in protostellar accretion disks and the development of their structure and chemical composition as the disk material slowly migrates inwards during the viscous phase of the disk evolution. It is shown that the amorphous structure of the dust grains from the parent molecular cloud is converted by annealing at about

H.-P. Gail

1998-01-01

249

Chemical reactions in protoplanetary accretion disks IV. Multicomponent dust mixture  

Microsoft Academic Search

We consider the different major components of the dust mixture in protostellar accretion disks and the development of their structure and chemical composition as the disk material slowly migrates inwards during the viscous phase of the disk evolution. It is shown that the amorphous structure of the dust grains from the parent molecular cloud is converted by anneal- ing at

H.-P. Gail

250

Single-molecule chemical reactions on DNA origami  

Microsoft Academic Search

DNA nanotechnology and particularly DNA origami, in which long, single-stranded DNA molecules are folded into predetermined shapes, can be used to form complex self-assembled nanostructures. Although DNA itself has limited chemical, optical or electronic functionality, DNA nanostructures can serve as templates for building materials with new functional properties. Relatively large nanocomponents such as nanoparticles and biomolecules can also be integrated

Niels V. Voigt; Thomas Tørring; Alexandru Rotaru; Mikkel F. Jacobsen; Jens B. Ravnsbæk; Ramesh Subramani; Wael Mamdouh; Jørgen Kjems; Andriy Mokhir; Flemming Besenbacher; Kurt Vesterager Gothelf

2010-01-01

251

Supercritical fluid phase separations induced by chemical reactions.  

National Technical Information Service (NTIS)

Our statistical mechanical studies predict that a chemically reactive system containing species composed of C, H, N, O atoms can exhibit a phase separation into a N2-rich and a N2-poor phase. This paper is concerned with the effect of the fluid phase sepa...

F. H. Ree, J. A. Viecelli, M. van Thiel

1997-01-01

252

Chemical-biological reactions common to teratogenesis and mutagenesis  

PubMed Central

Cytotoxic chemicals have in common the ability to act specifically on cells in cycle. Bacteria are more sensitive in the exponential growth phase than when growing slowly in media. Similar observations have been made on a variety of systems ranging from bacteria, yeast, higher plants and invertebrates to vertebrates including primates. The embryo and fetus are highly susceptible to cytotoxic agents because they have continuous groups of cells in the growth phase. Acutely toxic doses may cause cellular death and result in developmental defects or fetal death. Cytotoxic agents can be grouped as alkylating agents, electrophilic reactants, antimetabolites, intercalating agents, amino acid antagonists, spindle poisons, and an additional group of chemicals which covalently bind to DNA. These cytotoxic groups of chemicals may also be mutagenic by interacting with DNA to produce changes in sequences of nucleotides resulting in heritable defects either in a somatic cell line or in a germinal cell line. The mechanisms of chemical-induced teratogenicity and mutagenicity are similar. This commonality is further discussed in the text.

Harbison, Raymond D.

1978-01-01

253

X-ray Microspectroscopy and Chemical Reactions in Soil Microsites  

SciTech Connect

Soils provide long-term storage of environmental contaminants, which helps to protect water and air quality and diminishes negative impacts of contaminants on human and ecosystem health. Characterizing solid-phase chemical species in highly complex matrices is essential for developing principles that can be broadly applied to the wide range of notoriously heterogeneous soils occurring at the earth's surface. In the context of historical developments in soil analytical techniques, we describe applications of bulk-sample and spatially resolved synchrotron X-ray absorption spectroscopy (XAS) for characterizing chemical species of contaminants in soils, and for determining the uniqueness of trace-element reactivity in different soil microsites. Spatially resolved X-ray techniques provide opportunities for following chemical changes within soil microsites that serve as highly localized chemical micro- (or nano-)reactors of unique composition. An example of this microreactor concept is shown for micro-X-ray absorption near edge structure analysis of metal sulfide oxidation in a contaminated soil. One research challenge is to use information and principles developed from microscale soil chemistry for predicting macroscale and field-scale behavior of soil contaminants.

D Hesterberg; M Duff; J Dixon; M Vepraskas

2011-12-31

254

Experimental simulation of chemical reactions between ZDDP tribofilms and steel surfaces during friction processes  

Microsoft Academic Search

Friction tests are performed in a controlled environment (Ultra High Vacuum), between steel surfaces and a ZDDP tribofilm\\u000a at different contact severities. According to AES analyses, evidence of chemical reactions activated by friction is given.\\u000a The reaction of the ZDDP tribofilm with the native iron oxide could partially explain its antiwear behavior.

C. Minfray; T. Le Mogne; A. A. Lubrecht; J.-M. Martin

2006-01-01

255

Everyday Chemical Reactions: A Writing Assignment to Promote Synthesis of Concepts and Relevance in Chemistry  

Microsoft Academic Search

Capturing the interest of students in required chemistry courses is a problem for which many solutions have been proposed and described. The solution proposed here is the use of a writing assignment on everyday chemical reactions. Students select their own organic reaction and apply concepts learned throughout the semester to understanding it. The assignment requires the synthesis of many concepts

Abby L. Parrill

2000-01-01

256

Thermochemistry of the HOSO2+O2 association reaction and enthalpy of formation of HOSO4: A quantum chemical study  

NASA Astrophysics Data System (ADS)

The thermochemistry of the HOSO radical is of crucial importance for the modeling of the atmospheric sulfur-oxidation cycle, in which HOSO is formed in the reaction HOSO+O+M?HOSO+M(2a). We performed high-level quantum chemical calculations to characterize the energetics of reaction (2a) and obtained the following values for the enthalpy of reaction: ?H(2a) = -71.4 ± 2.0 and ?H(2a) = -72.9 ± 3.0 kJ/mol. For the enthalpy of formation of HOSO, it follows: ?H(HOSO)=-438.0±5.0kJ/mol and ?H(HOSO)=-447.0±6.0kJ/mol. These new thermochemical data can serve as a basis for kinetic studies of HOSO reactions in the atmosphere.

González-García, Núria; Klopper, Wim; Olzmann, Matthias

2009-02-01

257

Design of potential energy surfaces for chemical reactions  

SciTech Connect

The design of potential energy surfaces for two systems is considered. The first study presented is for the abstraction reaction F + H/sub 2/ ..-->.. HF + H and the exchange reaction H' + FH ..-->.. FH' + H and their related isotopic analogs. Several surfaces are proposed which incorporate both experimental and ab initio information. The new surfaces are of the form of the extended London-Eyring-Polanyi method but with angle-and distance-dependent Sato parameters and an added three-center term. The first new surface is fit using empirical data in the entrance channel but only ab initio data in the exit channel. Preliminary studies on the role of dynamical bottlenecks in the exit-channel are used to locate the crucial regions of the surface where accurate calculations are needed. A second surface is a slightly modified version of the first in which experimental information is used to recalibrate the abstraction exit channel. Two additional surfaces presented here are designed to test the effect of a higher entrance-channel saddle point on the dynamics. A study of potential energy surfaces for the reaction CH/sub 3/ + H/sub 2/ ..-->.. CH/sub 4/ + H is also presented. This includes a thorough analysis of existing surfaces and dynamics calculations using slightly modified forms of these surfaces. Two new surfaces are calibrated using both ab initio and experimental data. Rate constants, activation energies, and kinetic isotope effects calculated using the new surfaces are compared to experiment.

Steckler, R.

1986-01-01

258

On the feasibility of chemical reactions in the presence of siloxane-based surfactants  

Microsoft Academic Search

Siloxane-containing surfactants have been tested as stabilizers for the preparation of polymer nanoparticles by three types\\u000a of chemical reactions. Two crosslinking reactions were used to obtain silicone elastomers particles: one involved HO-terminated\\u000a polydimethylsiloxane and tetraethoxysilane, while the other one was a crosslinking via polyhydrosilylation. The third reaction\\u000a was a linear polycondensation between a diamine and a siloxane dialdehyde. The monitoring

Carmen Racles; Maria Cazacu; Gabriela Hitruc; Thierry Hamaide

2009-01-01

259

Site-selective reactions of imperfectly matched DNA with small chemical molecules: applications in mutation detection  

Microsoft Academic Search

The last decade has witnessed many exciting scientific publications associated with site-selective reactions of small chemical molecules with imperfectly matched DNA. Typical examples are carbodiimide, hydroxylamine, potassium permanganate, osmium tetroxide, chemical tagging probes, biotinylated, chemiluminescent and fluorescent probes, and all of them selectively react with imperfectly matched DNA. More recently, some therapeutic agents including DNA intercalating drugs and groove binders

Chinh T Bui; Kylee Rees; Andreana Lambrinakos; Abdulkerim Bedir; Richard G. H Cotton

2002-01-01

260

WORKSHOP ON STATUS OF TEST METHODS FOR ASSESSING POTENTIAL OF CHEMICALS TO INDUCE RESPIRATORY ALLERGIC REACTIONS  

EPA Science Inventory

Because of the association between allergy and asthma and the increasing incidence of morbidity and mortality due to asthma, there is growing concern over the potential of industrial chemicals to produce allergic reactions in the respiratory tract. Two classes of chemicals have b...

261

Indoor Volatile Organic Compounds and Chemical Sensitivity Reactions  

PubMed Central

Studies of unexplained symptoms observed in chemically sensitive subjects have increased the awareness of the relationship between neurological and immunological diseases due to exposure to volatile organic compounds (VOCs). However, there is no direct evidence that links exposure to low doses of VOCs and neurological and immunological dysfunction. We review animal model data to clarify the role of VOCs in neuroimmune interactions and discuss our recent studies that show a relationship between chronic exposure of C3H mice to low levels of formaldehyde and the induction of neural and immune dysfunction. We also consider the possible mechanisms by which VOC exposure can induce the symptoms presenting in patients with a multiple chemical sensitivity.

Win-Shwe, Tin-Tin; Arashidani, Keiichi; Kunugita, Naoki

2013-01-01

262

Influence of antioxidants on polyethylene chemical crosslinking reaction  

Microsoft Academic Search

The influence of antioxidants on the chemical crosslinking of crosslinked polyethylene (XLPE) was investigated. Experiments were conducted using XLPE specimens crosslinked at various temperatures with added phenolic, sulfur-type and amine-type antioxidants. Specimens were characterized by measuring the gel fraction and the content of crosslinking byproducts, acetophenone, cumyl alcohol and ?-methyl styrene. It was found that sulfur-type antioxidant has the strongest

Yasuo Sekii; Yuichi Idei; Kiyotaka Asakawa

2001-01-01

263

Chemical reactions for a deuteration network (Vastel+, 2012)  

NASA Astrophysics Data System (ADS)

The ground-state rotational ortho-D2H+(1,1,1-0,0,0) transition at 1476.6GHz in the prestellar core 16293E has been searched for with the Herschel/HIFI instrument, within the CHESS (Chemical HErschel Surveys of Star forming regions) Key Program. The line has not been detected at the 21mK.km/s level (3 sigma integrated line intensity). We used the ortho-H2D+ 110-111 transition and para-D2H+ 110-101 transition detected in this source to determine an upper limit on the ortho-to-para D2H+ ratio as well as the para-D2H+/ortho-H2D+ ratio from a non-LTE analysis. We then compared our chemical modeling with the observations in order to estimate the CO depletion as well as the H2 density and kinetic temperature at the position observed. The chemical network is provided in the kida.dat file. (1 data file).

Vastel, C.; Caselli, P.; Ceccarelli, C.; Bacmann, A.; Lis, D. C.; Caux, E.; Codella, C.; Beckwith, J. A.; Ridley, T.

2012-09-01

264

Wear mechanisms in metal-on-metal bearings: the importance of tribochemical reaction layers.  

PubMed

Metal-on-metal (MoM) bearings are at the forefront in hip resurfacing arthroplasty. Because of their good wear characteristics and design flexibility, MoM bearings are gaining wider acceptance with market share reaching nearly 10% worldwide. However, concerns remain regarding potential detrimental effects of metal particulates and ion release. Growing evidence is emerging that the local cell response is related to the amount of debris generated by these bearing couples. Thus, an urgent clinical need exists to delineate the mechanisms of debris generation to further reduce wear and its adverse effects. In this study, we investigated the microstructural and chemical composition of the tribochemical reaction layers forming at the contacting surfaces of metallic bearings during sliding motion. Using X-ray photoelectron spectroscopy and transmission electron microscopy with coupled energy dispersive X-ray and electron energy loss spectroscopy, we found that the tribolayers are nanocrystalline in structure, and that they incorporate organic material stemming from the synovial fluid. This process, which has been termed "mechanical mixing," changes the bearing surface of the uppermost 50 to 200 nm from pure metallic to an organic composite material. It hinders direct metal contact (thus preventing adhesion) and limits wear. This novel finding of a mechanically mixed zone of nanocrystalline metal and organic constituents provides the basis for understanding particle release and may help in identifying new strategies to reduce MoM wear. PMID:19877285

Wimmer, Markus A; Fischer, Alfons; Büscher, Robin; Pourzal, Robin; Sprecher, Christoph; Hauert, Roland; Jacobs, Joshua J

2010-04-01

265

Rate constants for chemical reactions in high-temperature nonequilibrium air  

NASA Technical Reports Server (NTRS)

In the nonequilibrium atmospheric chemistry regime that will be encountered by the proposed Aeroassisted Orbital Transfer Vehicle in the upper atmosphere, where air density is too low for thermal and chemical equilibrium to be maintained, the detailed high temperature air chemistry plays a critical role in defining radiative and convective heating loads. Although vibrational and electronic temperatures remain low (less than 15,000 K), rotational and translational temperatures may reach 50,000 K. Attention is presently given to the effects of multiple temperatures on the magnitudes of various chemical reaction rate constants, for the cases of both bimolecular exchange reactions and collisional excitation and dissociation reactions.

Jaffe, R. L.

1986-01-01

266

The mineralogic evolution of the Martian surface through time: Implications from chemical reaction path modeling studies  

NASA Technical Reports Server (NTRS)

Chemical reaction path calculations were used to model the minerals that might have formed at or near the Martian surface as a result of volcano or meteorite impact driven hydrothermal systems; weathering at the Martian surface during an early warm, wet climate; and near-zero or sub-zero C brine-regolith reactions in the current cold climate. Although the chemical reaction path calculations carried out do not define the exact mineralogical evolution of the Martian surface over time, they do place valuable geochemical constraints on the types of minerals that formed from an aqueous phase under various surficial and geochemically complex conditions.

Plumlee, G. S.; Ridley, W. I.; Debraal, J. D.; Reed, M. H.

1993-01-01

267

Rate-Controlled Constrained-Equilibrium Theory of Chemical Reactions  

NASA Astrophysics Data System (ADS)

The Rate-Controlled Constrained-Equilibrium (RCCE) method for simplifying the treatment of reactions in complex systems is summarized and the selection of constraints for both close-to and far-from equilibrium systems is discussed. Illustrative examples of RCCE calculations of carbon monoxide concentrations in the exhaust products of an internal combustion engine and ignition delays for methane-oxygen mixtures in a constant volume adiabatic chamber are given and compared with ``detailed'' calculations. The advantages of RCCE calculations over ``detailed'' calculations are discussed.

Keck, James C.

2008-08-01

268

Tracking chemical reactions on the surface of filamentous phage using mass spectrometry.  

PubMed

Chemical modification of phage libraries has allowed the in vitro evolution of ligands having properties not provided by natural polypeptides. The development of novel and more diverse chemical reactions on phage was hampered by the lack of analytical methods to efficiently monitor the reaction products on the more than 10?000 kDa large filamentous phage particles. Herein, we present a strategy to detect chemically modified peptides on phage based on enzymatic release of peptide from phage and mass spectrometry analysis. PMID:24291807

Chen, Shiyu; Touati, Jeremy; Heinis, Christian

2014-05-25

269

Kinetics of sulphuric alkylation reactions: study of two reaction steps and modelling using chemical families grouping.  

National Technical Information Service (NTIS)

Isobutane/olefins sulphuric alkylation is used in the petroleum industry to prepare fuels with high octane numbers. Although the alkylation reaction/process has been considered by numerous investigators, relatively few results have been published concerni...

L. E. Pizarro Borges

1995-01-01

270

15 CFR 713.3 - Annual declaration and reporting requirements for exports and imports of Schedule 2 chemicals.  

Code of Federal Regulations, 2010 CFR

...requirements for exports and imports of Schedule 2 chemicals. 713.3 Section 713.3 Commerce and Foreign Trade Regulations Relating...REGULATIONS ACTIVITIES INVOLVING SCHEDULE 2 CHEMICALS § 713.3 Annual declaration and reporting...

2010-01-01

271

15 CFR 713.3 - Annual declaration and reporting requirements for exports and imports of Schedule 2 chemicals.  

Code of Federal Regulations, 2010 CFR

...requirements for exports and imports of Schedule 2 chemicals. 713.3 Section 713.3 Commerce and Foreign Trade Regulations Relating...REGULATIONS ACTIVITIES INVOLVING SCHEDULE 2 CHEMICALS § 713.3 Annual declaration and reporting...

2009-01-01

272

Systematic trends in photonic reagent induced reactions in a homologous chemical family.  

PubMed

The growing use of ultrafast laser pulses to induce chemical reactions prompts consideration of these pulses as "photonic reagents" in analogy to chemical reagents. This work explores the prospect that photonic reagents may affect systematic trends in dissociative ionization reactions of a homologous family of halomethanes, much as systematic outcomes are often observed for reactions between homologous families of chemical reagents and chemical substrates. The experiments in this work with photonic reagents of varying pulse energy and linear spectral chirp reveal systematic correlations between observable ion yields and the following set of natural variables describing the substrate molecules: the ionization energy of the parent molecule, the appearance energy of each fragment ion, and the relative strength of carbon-halogen bonds in molecules containing two different halogens. The results suggest that reactions induced by photonic reagents exhibit systematic behavior analogous to that observed in reactions driven by chemical reagents, which provides a basis to consider empirical "rules" for predicting the outcomes of photonic reagent induced reactions. PMID:23909915

Tibbetts, Katharine Moore; Xing, Xi; Rabitz, Herschel

2013-08-29

273

High temperature chemical kinetic study of the H2-CO-CO2-NO reaction system  

NASA Technical Reports Server (NTRS)

An experimental study of the kinetics of the H2-CO-CO2-NO reaction system was made behind incident shock waves at temperatures of 2460 and 2950 K. The overall rate of the reaction was measured by monitoring radiation from the CO + O yields CO2 + h upoilon reaction. Correlation of these data with a detailed reaction mechanism showed that the high-temperature rate of the reaction N + OH yields NO + H can be described by the low-temperature (320 K) rate coefficient. Catalytic dissociation of molecular hydrogen was an important reaction under the tests conditions.

Jachimowski, C. J.

1975-01-01

274

Chemical reactions in the interstellar medium: The HOC+ ? HCO+ Isomerization Reaction catalyzed by H2  

NASA Astrophysics Data System (ADS)

The present work studies the catalytic effect of molecular hydrogen on the hydrogen transfer from O bounded to C bounded in HOC+ . For this purpose, the reactions with and without hydrogen assistance were studied. The potential energy surface of this reaction was studied using Density Functional Theory methods (PBE/TVZP). The energy barrier was found to be significantly lower than that of the isolated system: An energy profile in function of the intrinsic reaction coordinate was obtained and used to get the reaction force profile and reaction work, which in turn is used to characterize the reaction mechanism. The main contribution to the activation barrier in the uncatalyzed isomerization is W1= 24,42 kcal/mol and can be attributed to structural changes, which the system suffers in order to reach the transition state. This fact is not modified by the introduction of H2, where the main contribution again is W 1 = 8.38 kcal/mol.This shows that the hydrogen catalyst has no special effect over a specific region, but homogenously lowers W1 and W2.

Vogt-Geisse, S.; Toro-Labbe, A.

275

Coarse grain model for coupled thermo-mechano-chemical processes and its application to pressure-induced endothermic chemical reactions  

NASA Astrophysics Data System (ADS)

We extend a thermally accurate model for coarse grain dynamics (Strachan and Holian 2005 Phys. Rev. Lett. 94 014301) to enable the description of stress-induced chemical reactions in the degrees of freedom internal to the mesoparticles. Similar to the breathing sphere model, we introduce an additional variable that describes the internal state of the particles and whose dynamics is governed both by an internal potential energy function and by interparticle forces. The equations of motion of these new variables are derived from a Hamiltonian and the model exhibits two desired features: total energy conservation and Galilean invariance. We use a simple model material with pairwise interactions between particles and study pressure-induced chemical reactions induced by hydrostatic and uniaxial compression. These examples demonstrate the ability of the model to capture non-trivial processes including the interplay between mechanical, thermal and chemical processes of interest in many applications.

Antillon, Edwin; Banlusan, Kiettipong; Strachan, Alejandro

2014-03-01

276

Theoretical Chemical Dynamics Studies of Elementary Combustion Reactions  

SciTech Connect

The objective of this research was to develop and apply methods for more accurate predictions of reaction rates based on high-level quantum chemistry. We have developed and applied efficient, robust methods for fitting global ab initio potential energy surfaces (PESs) for both spectroscopy and dynamics calculations and for performing direct dynamics simulations. Our approach addresses the problem that high-level quantum calculations are often too costly in computer time for practical applications resulting in the use of levels of theory that are often inadequate for reactions. A critical objective was to develop practical methods that require the minimum number of electronic structure calculations for acceptable fidelity to the ab initio PES. Our method does this by a procedure that determines the optimal configurations at which ab initio points are computed, and that ensures that the final fitted PES is uniformly accurate to a prescribed tolerance. Our fitting methods can be done automatically, with little or no human intervention, and with no prior knowledge of the topology of the PES. The methods are based on local fitting schemes using interpolating moving least-squares (IMLS). IMLS has advantages over the very effective modified-Shepard methods developed by Collins and others in that higher-order polynomials can be used and does not require derivatives but can benefit from them if available.

Donald L. Thompson

2009-09-30

277

Quantum chemical study of the acrolein (CH2CHCHO) + OH + O2 reactions.  

PubMed

Acrolein, a beta-unsaturated (acrylic) aldehyde, is one of the simplest multifunctional molecules, containing both alkene and aldehyde groups. Acrolein is an atmospheric pollutant formed in the photochemical oxidation of the anthropogenic VOC 1,3-butadiene, and serves as a model compound for methacrolein (MACR) and methyl vinyl ketone (MVK), the major oxidation products of the biogenic VOC isoprene. In addition, acrolein is involved in combustion and biological oxidation processes. This study presents a comprehensive theoretical analysis of the acrolein + OH + O(2) addition reactions, which is a key photochemical oxidation sequence, using the G3SX and CBS-QB3 theoretical methods. Both ab initio protocols provide relatively similar results, although the CBS-QB3 method systematically under-predicts literature heats of formation using atomization enthalpies, and also provides lower transition state barrier heights. Several new low-energy pathways for unimolecular reaction of the acrolein-OH-O(2) radicals are identified, with energy at around or below that of the acrolein-OH isomers + O(2). In each case these novel reactions have the potential to reform the hydroxyl radical (OH) and form coproducts that include glyoxal, glycolaldehyde (HOCH(2)CHO), formaldehyde (HCHO), CO, and substituted epoxides. Analogous reaction schemes are developed for the photochemical oxidation of MACR and MVK, producing a number of observed oxidation products. The reaction MACR + OH + O(2) --> hydroxyacetone + OH + CO is expected to be of particular importance. This study also proposes that O(2) addition to chemically activated acrolein-OH adducts can provide prompt regeneration of OH in the atmospheric oxidation of acrolein, via a double activation mechanism. This mechanism can also be extended to isoprene, MVK, and MACR. The importance of the novel chemistry revealed here in the atmospheric oxidation of acrolein and other structurally related OVOCs and VOCs requires further investigation. Additionally, a critical evaluation of the acrolein heat of formation is presented, and a new value of -16.7 +/- 1.0 kcal mol(-1) is recommended along with other thermochemical properties, from a W1 level calculation. PMID:20701337

Asatryan, Rubik; da Silva, Gabriel; Bozzelli, Joseph W

2010-08-19

278

Direct imaging of covalent bond structure in single-molecule chemical reactions.  

PubMed

Observing the intricate chemical transformation of an individual molecule as it undergoes a complex reaction is a long-standing challenge in molecular imaging. Advances in scanning probe microscopy now provide the tools to visualize not only the frontier orbitals of chemical reaction partners and products, but their internal covalent bond configurations as well. We used noncontact atomic force microscopy to investigate reaction-induced changes in the detailed internal bond structure of individual oligo-(phenylene-1,2-ethynylenes) on a (100) oriented silver surface as they underwent a series of cyclization processes. Our images reveal the complex surface reaction mechanisms underlying thermally induced cyclization cascades of enediynes. Calculations using ab initio density functional theory provide additional support for the proposed reaction pathways. PMID:23722428

de Oteyza, Dimas G; Gorman, Patrick; Chen, Yen-Chia; Wickenburg, Sebastian; Riss, Alexander; Mowbray, Duncan J; Etkin, Grisha; Pedramrazi, Zahra; Tsai, Hsin-Zon; Rubio, Angel; Crommie, Michael F; Fischer, Felix R

2013-06-21

279

Out-of-equilibrium catalysis of chemical reactions by electronic tunnel currents  

NASA Astrophysics Data System (ADS)

We present an escape rate theory for current-induced chemical reactions. We use Keldysh nonequilibrium Green's functions to derive a Langevin equation for the reaction coordinate. Due to the out of equilibrium electronic degrees of freedom, the friction, noise, and effective temperature in the Langevin equation depend locally on the reaction coordinate. As an example, we consider the dissociation of diatomic molecules induced by the electronic current from a scanning tunnelling microscope tip. In the resonant tunnelling regime, the molecular dissociation involves two processes which are intricately interconnected: a modification of the potential energy barrier and heating of the molecule. The decrease of the molecular barrier (i.e., the current induced catalytic reduction of the barrier) accompanied by the appearance of the effective, reaction-coordinate-dependent temperature is an alternative mechanism for current-induced chemical reactions, which is distinctly different from the usual paradigm of pumping vibrational degrees of freedom.

Dzhioev, Alan A.; Kosov, Daniel S.; von Oppen, Felix

2013-04-01

280

Charge exchange and chemical reactions with trapped thorium ions  

NASA Astrophysics Data System (ADS)

Most atomic nuclei have excitation energies ranging from keV to MeV. A unique exception is the ^229Th nucleus, which has an excited state just several eV above the nuclear ground state.ootnotetextB. R. Beck et al., Phys. Rev. Lett. 98, 142501 (2007). Th^3+ provides a convenient level structure for laser cooling in an rf Paul trap.ootnotetextC. J. Campbell et al., Phys. Rev. Lett 102, 233004 (2009). Unlike many ions commonly utilized in precision measurements, the trap lifetime of Th^3+ is limited to only several minutes. This is a severe limitation to experiments involving ^229Th as it is only available in minute quantities. Here we have studied the loss mechanisms by introduction of various contaminants and analyzed reaction products using trapped ion mass spectrometry techniques.ootnotetextL. R. Churchill et al., Phys. Rev. A 83, 012710 (2011).

Depalatis, Michael; Churchill, Layne; Chapman, Michael

2011-06-01

281

Latest findings on the dynamics of the simplest chemical reaction  

NASA Astrophysics Data System (ADS)

This paper focuses on recent progress in the understanding of the H + H2 reaction and its isotopic variants. The detailed agreement between theory and experiment attained during the last years is emphasized and major experimental and theoretical advances are highlighted. The excellent description of most experimental findings, from state-resolved cross-sections to thermal rate constants, provided by the available quantum mechanical (QM) treatments, as well as the good overall behaviour of classical mechanics are underlined. Debated issues on short-lived complexes and delayed scattering, resonances and interferences, geometric-phase (GP) effects, or product rotational distributions are discussed. Finally, some prospects for future research on this prototypic system are presented.

Bañares, Luis; Aoiz, F. J.; Herrero, Victor J.

2006-01-01

282

Concentration fluctuations in a mesoscopic oscillating chemical reaction system  

NASA Astrophysics Data System (ADS)

Under sustained pumping, kinetics of macroscopic nonlinear biochemical reaction systems far from equilibrium either can be in a stationary steady state or can execute sustained oscillations about a fixed mean. For a system of two dynamic species X and Y, the concentrations nx and ny will be constant or will repetitively trace a closed loop in the (nx, ny) phase plane, respectively. We study a mesoscopic system with nx and ny very small; hence the occurrence of random fluctuations modifies the deterministic behavior and the law of mass action is replaced by a stochastic model. We show that nx and ny execute cyclic random walks in the (nx, ny) plane whether or not the deterministic kinetics for the corresponding macroscopic system represents a steady or an oscillating state. Probability distributions and correlation functions for nx(t) and ny(t) show quantitative but not qualitative differences between states that would appear as either oscillating or steady in the corresponding macroscopic systems. A diffusion-like equation for probability P(nx, ny, t) is obtained for the two-dimensional Brownian motion in the (nx, ny) phase plane. In the limit of large nx, ny, the deterministic nonlinear kinetics derived from mass action is recovered. The nature of large fluctuations in an oscillating nonequilibrium system and the conceptual difference between "thermal stochasticity" and "temporal complexity" are clarified by this analysis. This result is relevant to fluorescence correlation spectroscopy and metabolic reaction networks. fluorescence correlation spectroscopy | limit cycle | nanobiochemistry | nonequilibrium steady state | random walk

Qian, Hong; Saffarian, Saveez; Elson, Elliot L.

2002-08-01

283

Quantum chemical approach to study the spectral properties of some important precursor of bio-molecules  

NASA Astrophysics Data System (ADS)

This work reports the spectral information of some interstellar complex molecules which could be treated as the precursor molecules for the formation of some bio-molecules in the interstellar medium (ISM). We carry out quantum chemical simulation to consider the Core correlation and vibrational corrections to the rotational constants and centrifugal distortion constants which are computed from harmonic and anharmonic force fields obtained at MP2/6-311G(d,p) level of theory. These precursor molecules could be produced in the gas phase as well as in the ice phase. This prompted use to couple the hydrodynamics of the collapsing phase of the protostar with our reasonably large chemical network including the gas phase as well as the grain phase chemical network, to study the chemical evolution of these species during the collapsing phase of a proto-star. We have noticed the Significant differences between spectroscopy of these species in the gas as well as in ice (water ice) phase due to the solute-solvent elctrostatic interactions. Time dependent density functional theory (TDDFT) is used to study the UV-VIS spectrum of these complexmolecules which are biologically important. Interstellar grain mantle around the dense cloud (> 104 cm-3) mainly composed by 60-70% Water, 5-30% Methanol and 2-20% CO2. So in reality, the ice could be mixed instead of simple water ice. To have an idea about the real spectra, we carry out our simulations for the mixed ice also by considering the actual composition of the above species in interstellar grain mantle. Spectral signatures are found to be significantly shifted with change of the solvent which confirms that the polarization of the solute by the continum has important effects on the absolute and relative solvation energies. Our simulated spectrum are in good agreement with some of the recent experimental result. We expect that our quantum chemical approach along with the hydro-chemical study might be useful for the observer to predict the abundances of some bio-molecules based on the chemical abundances of their precursor molecules.

Majumdar, Liton; Das, Ankan; Chakrabarti, Sandip K.; Chakrabarti, Sonali

2013-06-01

284

Molecular beam studies of hot atom chemical reactions: Reactive scattering of energetic deuterium atoms  

SciTech Connect

A brief review of the application of the crossed molecular beams technique to the study of hot atom chemical reactions in the last twenty years is given. Specific emphasis is placed on recent advances in the use of photolytically produced energetic deuterium atoms in the study of the fundamental elementary reactions D + H/sub 2/ /minus/> DH + H and the substitution reaction D + C/sub 2/H/sub 2/ /minus/> C/sub 2/HD + H. Recent advances in uv laser and pulsed molecular beam techniques have made the detailed study of hydrogen atom reactions under single collision conditions possible. 18 refs., 9 figs.

Continetti, R.E.; Balko, B.A.; Lee, Y.T.

1989-02-01

285

Nonequilibrium effects on the rate of bimolecular chemical reaction in a dilute gas  

NASA Astrophysics Data System (ADS)

The perturbation solution of the Boltzmann equation in a dilute gas shows that the rate constant of chemical reaction A+ A ? B+ B is diminished due to nonequilibrium effects. For the line-of-centers model, the relative decrease ? of the reaction rate can reach even nearly 45% (for a small reduced threshold energy ?* and a large molar fraction xB). For slow reactions this method is valid and ? does not depend on xB. The appropriate equilibrium expressions for reaction rate (with: (a) the temperature of the system T, (b) the nonequilibrium Shizgal-Karplus temperatures TA and TB) are used for this analysis.

Cukrowski, A. S.; Fritzsche, S.; Fort, J.

2001-06-01

286

Child-Rearing Practices toward Children with Hemophilia: The Relative Importance of Clinical Characteristics and Parental Emotional Reactions.  

ERIC Educational Resources Information Center

Addresses the relative importance of clinical characteristics of the child and parental emotional reactions, to child-rearing practices towards children with hemophilia. Results indicate that mother's emotional reactions appear to have a stronger influence on child-rearing uncertainty and overprotection than clinical characteristics of the child.…

Banis, S.; Suurmeijer, Th. P. B. M.; van Peer, D. R.

1999-01-01

287

Spectator Ions ARE Important! A Kinetic Study of the Copper-Aluminum Displacement Reaction  

ERIC Educational Resources Information Center

Surprisingly, spectator ions are responsible for unexpected kinetics in the biphasic copper(II)-aluminum displacement reaction, with the rate of reaction dependent on the identity of the otherwise ignored spectator ions. Application of a published kinetic analysis developed for a reaction between a rotating Al disk and a Cu(II) ion solution to the…

Sobel, Sabrina G.; Cohen, Skyler

2010-01-01

288

Influence of reaction heat on time dependent processes in a chemically reacting binary mixture  

NASA Astrophysics Data System (ADS)

In this paper we study time dependent problems, like the propagation of sound waves or the behavior of small local wave disturbances induced by spontaneous internal fluctuations, in a binary mixture undergoing a chemical reaction of type A + A ? B + B. The study is developed at the hydrodynamic Euler level, in a chemical regime of fast reactive process in which the chemical reaction is close to its final equilibrium state. The hydrodynamic state of the mixture is described by the balance equations for the mass densities of both constituents A and B, together with the conservation laws for the momentum and total energy of the mixture. The progress of the chemical reaction is specified by an Arrhenius-type reaction rate which defines the net balance between production and consumption of each constituent. Assuming that the considered time dependent problems induce weak macroscopic deviations, the hydrodynamic equations are linearized through a normal mode expansion of the state variables around the equilibrium state. From the dispersion relation of the normal modes, we determine the free and forced phase velocities as well as the attenuation coefficients of the waves. We show that the dispersion and absorption of these waves depend explicitly on the heat of the chemical reaction, the concentrations of the constituents and the activation energy through the exponential factor of Arrhenius law.

Marques, Wilson, Jr.; Kremer, Gilberto M.; Soares, Ana Jacinta

2012-11-01

289

Laboratory Studies of Sticking Coefficients and Heterogeneous Reactions Important in the Stratosphere.  

National Technical Information Service (NTIS)

The discovery of ozone depletion during spring in the Antarctic stratosphere has received wide spread attention. Both meteorological and chemical mechanisms have been used in attempts to explain this observation. The chemical theory focused on the chlorof...

M. Leu

1988-01-01

290

Chemical reactions of ultracold alkali-metal dimers in the lowest-energy 3? state  

NASA Astrophysics Data System (ADS)

We show that the interaction of polar alkali-metal dimers in the quintet spin state leads to the formation of a deeply bound reaction complex. The reaction complex can decompose adiabatically into homonuclear alkali-metal dimers (for all molecules except KRb) and into alkali-metal trimers (for all molecules). We show that there are no barriers for these chemical reactions. This means that all alkali-metal dimers in the a3?+ state are chemically unstable at ultracold temperature, and the use of an optical lattice to segregate the molecules and suppress losses may be necessary. In addition, we calculate the minimum-energy path for the chemical reactions of alkali-metal hydrides. We find that the reaction of two molecules is accelerated by a strong attraction between the alkali-metal atoms, leading to a barrierless process that produces hydrogen atoms with large kinetic energy. We discuss the unique features of the chemical reactions of ultracold alkali-metal dimers in the a3?+ electronic state.

Tomza, Micha?; Madison, Kirk W.; Moszynski, Robert; Krems, Roman V.

2013-11-01

291

Computed potential energy surfaces for chemical reactions. Semiannual report, 1 January-30 June 1990  

SciTech Connect

The objective was to obtain accurate potential energy surfaces (PES's) for a number of reactions which are important in the H/N/O combustion process. The interest in this is centered around the design of the SCRAM jet engine for the National Aerospace Plane (NASP), which was envisioned as an air-breathing hydrogen-burning vehicle capable of reaching velocities as large as Mach 25. Preliminary studies indicated that the supersonic flow in the combustor region of the scram jet engine required accurate reaction rate data for reactions in the H/N/O system, some of which was not readily available from experiment. The most important class of combustion reactions from the standpoint of the NASP project are radical recombinaton reactions, since these reactions result in most of the heat release in the combustion process. Theoretical characterizations of the potential energy surfaces for these reactions are presented and discussed.

Walch, S.P.

1990-01-01

292

[Study on the chemical reaction of DNA with Congo red].  

PubMed

The interaction of deoxyribonucleic acid(DNA) and Congo red (GGH) was investigated by UV-Vis spectrophotometry in Tris-HCl solution (pH 4.56). When DNA was added into GGH solution, the color of the system changed from red to purple, which indicated the formation of the DNA-GGH complex. The maximum absorption of the complex is at 600 nm. The molar absorptivity measured at this wavelength epsilon = 1.41 x 10(5) L x cm(-1) mol(-1), the maximum binding number is n = 32, and the detection limit is c = 8.04 x 10(-8) mol x L(-1). The basic reaction condition of best pH value, time, and temperature, and the interference of different materials on the system were also studied. The ionic strength could affect the absorption of the system. The interaction of small molecule and DNA, the molecule structure, and the relationship between the molecule conformation and the distribution of electron cloud were studied. PMID:16830404

Chi, Yan-hua; Zhuang, Jia; Xue, Qi-bin; Li, Ke-an

2006-01-01

293

Analysis of Nonequilibrium Effects in a Bimolecular Chemical Reaction in a Dilute Gas  

NASA Astrophysics Data System (ADS)

We analyze a thermally activated bimolecular reaction in a dilute gas proceeding with introduction of the Prigogine--Xhrouet model (PX) for the reactive cross section. We use the Shizgal--Karplus perturbation method of solution of the Boltzmann equation for reactions A + A ? B + B and A + A ? B + B to obtain the analytical expressions for the nonequilibrium temperatures of reagents and for the rate of chemical reaction. We present the results obtained within one and two Sonine polynomials approximations. The rate constant of chemical reaction depends on concentration of products for the first reaction only and for the second reaction is constant. The analytical results for the temperature of the reagent A and its value in the beginning of reaction for the product B are compared with those obtained from the Monte Carlo computer simulations with use of the Bird method. It is shown that the nonequilibrium effects in Shizgal--Karplus temperatures and in decrease of the chemical constant rate are more pronounced than for the lines-of-centers model. For the PX model the rate constant can be decreased even 4 times.

Cukrowski, Andrzej S.; Fritzsche, Siegfried

2003-07-01

294

Kilometer-scale chemical reaction boundary patterns and deformation in sedimentary rocks  

NASA Astrophysics Data System (ADS)

We use three-dimensional (3D) seismic data to image patterns developed by kilometer-scale chemical reaction boundaries to demonstrate that chemical patterns are scaleable phenomena. The patterns develop in biosiliceous marine successions due to the dissolution of opal-A (biogenic silica) and reprecipitation as opal-CT (Crystobalite and Tridymite) during burial. The reaction boundary patterns comprise roughly circular regions where the reaction boundary has preferentially advanced. These regions are up to 2.7 km wide, and c. 50-200 m in height and are termed cells. The cells form by amalgamation with adjacent juvenile cells. They also form by the incorporation of much smaller regions surrounding the cells where the chemical change has already occurred that we term 'satellites' (50 m wide). The reaction results in enhanced rates of sediment compaction hence differential advancement of reaction boundaries causes differential subsidence of the overburden, inducing folding and faulting of the overburden above the cellular promontories. We propose three potential mechanisms for the development of kilometer-scale reaction boundary patterns: (a) mass transport of silica by advection (b) perturbation of isotherms as a result of convective or conductive heat transport or (c) establishment of a positive feedback loop between fluid production due to the reaction, hydraulic fracturing and the upward and lateral transport of fluids. This study provides the first insights into how strongly patterned diagenetic reaction boundaries evolve at a basin scale, an initial conceptualization of the potential range of reaction boundary morphologies that could exist in this diagenetic system, and the likely mechanisms that could control them. Furthermore it demonstrates that the discipline of 'seismic diagenesis' could represent a completely new approach for the study of chemical diagenetic processes in sedimentary rocks.

Davies, Richard J.; Cartwright, Joe A.

2007-10-01

295

Chemically activated formation of organic acids in reactions of the Criegee intermediate with aldehydes and ketones.  

PubMed

Reactions of the Criegee intermediate (CI, ?CH2OO?) are important in atmospheric ozonolysis models. In this work, we compute the rates for reactions between ?CH2OO? and HCHO, CH3CHO and CH3COCH3 leading to the formation of secondary ozonides (SOZ) and organic acids. Relative to infinitely separated reactants, the SOZ in all three cases is found to be 48-51 kcal mol(-1) lower in energy, formed via 1,3-cycloaddition of ?CH2OO? across the C=O bond. The lowest energy pathway found for SOZ decomposition is intramolecular disproportionation of the singlet biradical intermediate formed from cleavage of the O-O bond to form hydroxyalkyl esters. These hydroxyalkyl esters undergo concerted decomposition providing a low energy pathway from SOZ to acids. Geometries and frequencies of all stationary points were obtained using the B3LYP/MG3S DFT model chemistry, and energies were refined using RCCSD(T)-F12a/cc-pVTZ-F12 single-point calculations. RRKM calculations were used to obtain microcanonical rate coefficients (k(E)) and the reservoir state method was used to obtain temperature and pressure dependent rate coefficients (k(T, P)) and product branching ratios. At atmospheric pressure, the yield of collisionally stabilized SOZ was found to increase in the order HCHO < CH3CHO < CH3COCH3 (the highest yield being 10(-4) times lower than the initial ?CH2OO? concentration). At low pressures, chemically activated formation of organic acids (formic acid in the case of HCHO and CH3COCH3, formic and acetic acid in the case of CH3CHO) was found to be the major product channel in agreement with recent direct measurements. Collisional energy transfer parameters and the barrier heights for SOZ reactions were found to be the most sensitive parameters determining SOZ and organic acid yield. PMID:23958859

Jalan, Amrit; Allen, Joshua W; Green, William H

2013-10-21

296

Differential susceptibility of PCR reactions to inhibitors: an important and unrecognised phenomenon  

PubMed Central

Background PCR inhibition by nucleic acid extracts is a well known yet poorly described phenomenon. Inhibition assessment generally depends on the assumption that inhibitors affect all PCR reactions to the same extent; i.e. that the reaction of interest and the control reaction are equally susceptible to inhibition. To test this assumption we performed inhibition assessment on DNA extracts from human urine samples, fresh urine and EDTA using different PCR reactions. Results When copurified inhibitors were assessed using two different PCR reactions one reaction appeared to be inhibited whilst the other was not. Further experiments using various concentrations of unextracted urine to inhibit six different PCR reactions revealed that susceptibility to inhibition was highly variable between reactions. Similar results were obtained using EDTA as the PCR inhibitor. We could find no obvious explanation why one reaction should be more susceptible to inhibition than another, although a possible association with amplicon GC content was noted. Conclusion These findings have serious implications for all PCR-based gene expression studies, including the relatively new PCR array method, and for both qualitative and quantitative PCR-based molecular diagnostic assays, suggesting that careful consideration should be given to inhibition compatibility when conducting PCR analyses. We have demonstrated unequivocally that it is not safe to assume that different PCR reactions are equally susceptible to inhibition by substances co-purified in nucleic acid extracts.

Huggett, Jim F; Novak, Tanya; Garson, Jeremy A; Green, Clare; Morris-Jones, Stephen D; Miller, Robert F; Zumla, Alimuddin

2008-01-01

297

Coupling quantum interpretative techniques: another look at chemical mechanisms in organic reactions.  

PubMed

A cross ELF-NCI analysis is tested over prototypical organic reactions. The synergetic use of ELF and NCI enables the understanding of reaction mechanisms since each method can respectively identify regions of strong and weak electron pairing. Chemically intuitive results are recovered and enriched by the identification of new features. Non covalent interactions are found to foresee the evolution of the reaction from the initial steps. Within NCI, no topological catastrophe is observed as changes are continuous to such an extent that future reaction steps can be predicted from the evolution of the initial NCI critical points. Indeed, strong convergences through the reaction paths between ELF and NCI critical points enable to identify key interactions at the origin of the bond formation. VMD scripts enabling the automatic generation of movies depicting the cross NCI/ELF analysis along a reaction path (or following a Born-Oppenheimer molecular dynamics trajectory) are provided as S.I. PMID:23185140

Gillet, Natacha; Chaudret, Robin; Contreras-Garc?a, Julia; Yang, Weitao; Silvi, Bernard; Piquemal, Jean-Philip

2012-11-13

298

Importance of turbulent mixing and advective transport for the chemical evolution of protoplanetary disks  

NASA Astrophysics Data System (ADS)

The understanding of the planet formation scenario requires an in-depth knowledge about the evolution of protoplanetary disks. The key process for the disk evolution is the accretion of the disk matter on a central star, which is caused by the redistribution of the angular momentum due to anomalous viscosity driven by the turbulence. While the origin of the turbulence is now close to be understood (e.g., Balbus & Hawley 1991), its basic properties, like efficiency of diffusive mixing, still remain uncertain (e.g., Brandenburg & Sokoloff 2002, Johansen & Klahr 2005). On the other hand, the global transport processes in protoplanetary disks are relatively well studied the bulk of the disk matter adjacent to midplane moves inward, thus, carrying the mass, while the rest of the disk material moves outward in the upper layer, thus, carrying the angular momentum (e.g., Keller & Gail 2004). Consequently, one may expect that the turbulent diffusion and the global transport of the matter affect to a high extent the chemical processes leading to the formation and destruction of molecules in protoplanetary disks. To study this effect, we use a 2D flared disk model with vertical temperature gradient, a 2D advection-diffusion gas-grain chemical model supplied with surface reactions, and a 2D line radiative transfer code. Using these tools, we calculate time-dependent molecular abundances, column densities, and single-dish line profiles. These results are compared with those from a static (non-mixing, non-advective) chemical model and observational data. A special emphasis is placed on the identification of the observable quantities that can be used as direct indicators for the efficiency of the turbulent mixing and advective processes in protoplanetary disks.

Semenov, Dmitry A.; Wiebe, Dmitry S.; Henning, Thomas K.; Pavlyuchenkov, Yaroslav N.

299

The Role of Comprehensive Detailed Chemical Kinetic Reaction Mechanisms in Combustion Research  

SciTech Connect

Recent developments by the authors in the field of comprehensive detailed chemical kinetic reaction mechanisms for hydrocarbon fuels are reviewed. Examples are given of how these mechanisms provide fundamental chemical insights into a range of combustion applications. Practical combustion consists primarily of chemical heat release from reactions between a fuel and an oxidizer, and computer simulations of practical combustion systems have become an essential tool of combustion research (Westbrook et al., 2005). At the heart of most combustion simulations, the chemical kinetic submodel frequently is the most detailed, complex and computationally costly part of a system model. Historically, the chemical submodel equations are solved using time-implicit numerical algorithms, due to the extreme stiffness of the coupled rate equations, with a computational cost that varies roughly with the cube of the number of chemical species in the model. While early mechanisms (c. 1980) for apparently simple fuels such as methane (Warnatz, 1980) or methanol (Westbrook and Dryer, 1979) included perhaps 25 species, current detailed mechanisms for much larger, more complex fuels such as hexadecane (Fournet et al., 2001; Ristori et al., 2001; Westbrook et al., 2008) or methyl ester methyl decanoate (Herbinet et al., 2008) have as many as 2000 or even 3000 species. Rapid growth in capabilities of modern computers has been an essential feature in this rapid growth in the size and complexity of chemical kinetic reaction mechanisms.

Westbrook, C K; Pitz, W J; Curran, H J; Mehl, M

2008-07-16

300

Can information of chemical reaction propagate with plasmonic waveguide and be detected at remote terminal of nanowire?  

NASA Astrophysics Data System (ADS)

We attempt to provide experimental and theoretical evidence that information of chemical reaction can propagate with plasmonic waveguide along the nanowire and be detected at the remote terminal of nanowire, where the chemical reaction is the surface catalyzed reaction of DMAB produced from PATP assisted by surface plasmon polaritons.

Sun, Mengtao; Hou, Yanxue; Xu, Hongxing

2011-10-01

301

Reaction and spontaneity: the influence of meaning from everyday language on fourth year undergraduates’ interpretations of some simple chemical phenomena  

Microsoft Academic Search

Fourteen fourth?year undergraduate chemistry students were interviewed in the context of some simple chemical phenomena to find out their concepts of ‘reaction’ and ‘spontaneous’. The students were asked to discuss and to decide in four situations whether or not a chemical reaction had taken place and then, if one had, to decide whether or not the reaction was spontaneous. The

M. Gabriela; T. C. Ribeiro; Duarte J. V. Costa Pereira; Roger Maskill

1990-01-01

302

Importance of the Voronoi domain partition for position-jump reaction-diffusion processes on nonuniform rectilinear lattices  

NASA Astrophysics Data System (ADS)

Position-jump processes are used for the mathematical modeling of spatially extended chemical and biological systems with increasing frequency. A large subset of the literature concerning such processes is concerned with modeling the effect of stochasticity on reaction-diffusion systems. Traditionally, computational domains have been divided into regular voxels. Molecules are assumed well mixed within each of these voxels and are allowed to react with other molecules within the same voxel or to jump to neighboring voxels with predefined transition rates. For a variety of reasons implementing position-jump processes on irregular grids is becoming increasingly important. However, it is not immediately clear what form an appropriate irregular partition of the domain should take if it is to allow the derivation of mean molecular concentrations that agree with a given partial differential equation for molecular concentrations. It has been demonstrated, in one dimension, that the Voronoi domain partition is the appropriate method with which to divide the computational domain. In this Brief Report, we investigate theoretically the propriety of the Voronoi domain partition as an appropriate method to partition domains for position-jump models in higher dimensions. We also provide simulations of diffusion processes in two dimensions in order to corroborate our results.

Yates, Christian A.; Baker, Ruth E.

2013-11-01

303

Importance of the Voronoi domain partition for position-jump reaction-diffusion processes on nonuniform rectilinear lattices.  

PubMed

Position-jump processes are used for the mathematical modeling of spatially extended chemical and biological systems with increasing frequency. A large subset of the literature concerning such processes is concerned with modeling the effect of stochasticity on reaction-diffusion systems. Traditionally, computational domains have been divided into regular voxels. Molecules are assumed well mixed within each of these voxels and are allowed to react with other molecules within the same voxel or to jump to neighboring voxels with predefined transition rates. For a variety of reasons implementing position-jump processes on irregular grids is becoming increasingly important. However, it is not immediately clear what form an appropriate irregular partition of the domain should take if it is to allow the derivation of mean molecular concentrations that agree with a given partial differential equation for molecular concentrations. It has been demonstrated, in one dimension, that the Voronoi domain partition is the appropriate method with which to divide the computational domain. In this Brief Report, we investigate theoretically the propriety of the Voronoi domain partition as an appropriate method to partition domains for position-jump models in higher dimensions. We also provide simulations of diffusion processes in two dimensions in order to corroborate our results. PMID:24329390

Yates, Christian A; Baker, Ruth E

2013-11-01

304

Competition between charge exchange and chemical reaction - The D2/+/ + H system  

NASA Technical Reports Server (NTRS)

Study of the special features of molecular charge exchange and its competition with chemical reaction in the case of the D2(+) + H system. The trajectory surface hopping (TSH) model proposed by Tully and Preston (1971) is used to study this competition for a number of reactions involving the above system. The diatomics-in-molecules zero-overlap approximation is used to calculate the three adiabatic surfaces - one triplet and two singlet - which are needed to describe this system. One of the significant results of this study is that the chemical reaction and charge exchange are strongly coupled. It is also found that the number of trajectories passing into the chemical regions of the three surfaces depends very strongly on the surface crossings.-

Preston, R. K.; Cross, R. J., Jr.

1973-01-01

305

Simple Models for Nonequilibrium Effects in Bimolecular Chemical Reaction in a Dilute Gas  

NASA Astrophysics Data System (ADS)

Two models for reactive cross sections are introduced to analyze non-equilibrium effects connected with proceeding of the bimolecular chemical reaction A+A ? B+B in a dilute gas: 1. Line-of-Centers model LC, 2. the reverse model rLC leading to negative values of the Arrhenius activation energy. The perturbation method of solution of the Boltzmann equation is used to obtain analytical expressions for the rate constant of chemical reaction and for the nonequilibrium Shizgal--Karplus temperatures. It is shown that if the molar fraction of product is large enough the relative change of the rate of chemical reaction is constant, i.e. does not depend on the molar fraction. Replacing the equilibrium temperature by the nonequilibrium one (depending on the molar fraction) in the equilibrium equations for forward and reverse rate constants confirms these results.

Cukrowski, Andrzej S.; Fort, Joaquim; Fritzsche, Siegfried

2002-04-01

306

Optical thin films grown by surface chemical reaction for high-power lasers  

NASA Astrophysics Data System (ADS)

We developed a novel coating method using chemical reactions of gaseous reactants on a surface. A self-limiting nature of surface chemical reactions allows precisely controlled growth of films with high uniformity and controllability of thickness over large area. The nonuniformity of thickness distribution was under 1% over 240 mm in diameter. The film thickness had proportional relationship with a number of chemical reactions. TiO2 films at growth temperature of 25 degree(s)C had a laser-induced damage threshold of 5 J/cm2 for 1-ns, 1.06-micrometers laser pulses. The laser damage resistance of TiO2 films decreased at higher growth temperature. TiO2 films grown at the high temperature had higher crystallinity. We clarified that the laser damages resulted from the local sites that absorbed the laser energy.

Zaitsu, Shin-ichi; Motokoshi, Shinji; Jitsuno, Takahisa; Nakatsuka, Masahiro; Yamanaka, Tatsuhiko

2001-04-01

307

On the curvature in logarithmic plots of rate coefficients for chemical reactions  

PubMed Central

In terms of the reduced potential energy barrier ? = ?uTS/kT, the rate coefficients for chemical reactions are usually expressed as proportional to e-?. The coupling between vibrational modes of the medium to the reaction coordinate leads to a proportionality of the regularized gamma function of Euler Q(a,?) = ?(a,?)/?(a), with a being the number of modes coupled to the reaction coordinate. In this work, the experimental rate coefficients at various temperatures for several chemical reactions were fitted to the theoretical expression in terms of Q(a,?) to determine the extent of its validity and generality. The new expression affords lower deviations from the experimental points in 29 cases out of 38 and it accounts for the curvature in the logarithmic plots of rate coefficients versus inverse temperature. In the absence of tunneling, conventional theories predict the curvature of these plots to be identically zero.

2011-01-01

308

Chemical Synthesis of the Lantibiotic Lacticin 481 Reveals the Importance of Lanthionine Stereochemistry  

PubMed Central

Lantibiotics are a family of antibacterial peptide natural products characterized by the post-translational installation of the thioether-containing amino acids lanthionine and methyllanthionine. Until recently, only a single naturally occurring stereochemical configuration for each of these cross-links was known. The discovery of lantibiotics with alternative lanthionine and methyllanthionine stereochemistry has prompted an investigation of its importance to biological activity. Here, solid-supported chemical synthesis enabled the total synthesis of the lantibiotic lacticin 481 and analogues containing cross-links with non-native stereochemical configurations. Biological evaluation revealed that these alterations abolished the antibacterial activity in all of the analogues, revealing the critical importance of the enzymatically installed stereochemistry for the biological activity of lacticin 481.

2013-01-01

309

Chemical additives to control expansion of alkali-silica reaction gel: proposed mechanisms of control  

Microsoft Academic Search

Calcium chloride, lithium chloride, and acetone have previously been shown to affect expansion caused by alkali-silica reaction (ASR), a deleterious reaction occurring between reactive siliceous minerals present in some aggregate and the strongly alkaline pore solution in concrete. Here, the effect of these chemical additives was examined by transmission soft X-ray microscopy and a quantitative elemental analysis, using ICP-OES. In

K. E. Kurtis; P. J. M. Monteiro

2003-01-01

310

A free boundary problem describing reaction–diffusion problems in chemical vapor infiltration of pyrolytic carbon  

Microsoft Academic Search

In this paper we consider chemical vapor deposition of pyrolytic carbon from methane in hot wall reactors. Especially, we deal with the interaction of homogeneous gas-phase and heterogeneous surface reactions. The resulting mathematical model is composed of a system of reaction–diffusion equations in a corner domain supplied with the Gibbs–Thomson law, which describes the movement of the free boundary, arising

W. Merz; P. Rybka

2004-01-01

311

Diffusion-controlled chemical reactions modeled by continuous-time random walks  

Microsoft Academic Search

In the kinetic theory of pseudo-first-order diffusion-controlled chemical reactions, the diffusion motion of one component can be suppressed as long as the relative diffusion coefficient is assigned to the other component. The continuous-time-random-walk (CTWR) treatment of the pseudo-first-order reactions reaffirmed this well-known notion in the case of an exponential jump time distribution. In the case of a long-tail jump time

W. P. Helman; K. Funabashi

1979-01-01

312

Chemical modelling of Alkali Silica reaction: Influence of the reactive aggregate size distribution  

Microsoft Academic Search

This article presents a new model which aims at predicting the expansion induced by Alkali Silica Reaction (ASR) and describing\\u000a the chemical evolution of affected concretes. It is based on the description of the transport and reaction of alkalis and\\u000a calcium ions within a Relative Elementary Volume (REV). It takes into account the influence of the reactive aggregate size\\u000a grading

S. Poyet; A. Sellier; B. Capra; G. Foray; J.-M. Torrenti; H. Cognon; E. Bourdarot

2007-01-01

313

Simulation of Chemical Isomerization Reaction Dynamics on a NMR Quantum Simulator  

NASA Astrophysics Data System (ADS)

Quantum simulation can beat current classical computers with minimally a few tens of qubits. Here we report an experimental demonstration that a small nuclear-magnetic-resonance quantum simulator is already able to simulate the dynamics of a prototype laser-driven isomerization reaction using engineered quantum control pulses. The experimental results agree well with classical simulations. We conclude that the quantum simulation of chemical reaction dynamics not computable on current classical computers is feasible in the near future.

Lu, Dawei; Xu, Nanyang; Xu, Ruixue; Chen, Hongwei; Gong, Jiangbin; Peng, Xinhua; Du, Jiangfeng

2011-07-01

314

Optimal path of the consecutive chemical reactions xA?yB?zC  

NASA Astrophysics Data System (ADS)

The optimal path of the consecutive chemical reactions xA?yB?zC (x, y and z are the orders of chemical reactions) is analyzed numerically by taking temperature as a control variable, using the optimal-control theory based on Bak et al's work (2002 J. Phys. Chem. A 106 10961-4). Starting with pure A and maximizing the yield of B at the end of the given process duration, the optimal path starts with a branch at infinite temperature. A curve in which switching from this temperature to a lower temperature is possible is derived. For given parameters, there is a unique 'maximal useful time' that results in the highest possible yield of B. If a duration longer than this is specified, all reactions should be shut off during that excess amount of time in the optimal path. A numerical example for the optimal path is provided by the Taylor method. Finally, the results obtained are compared with the results of A?B?C (the orders of chemical reactions are all equal to 1) (Bak et al 2002 J. Phys. Chem. A 106 10961-4). When the orders of chemical reactions are taken into account, the optimal time sequences of the concentrations change markedly, the maximum obtainable yield is smaller, the initial values of co-state variables are bigger and the optimal phase trajectory changes.

Chen, Lingen; Song, Hanjiang; Sun, Fengrui

2009-05-01

315

Persistence of transition-state structure in chemical reactions driven by fields oscillating in time  

NASA Astrophysics Data System (ADS)

Chemical reactions subjected to time-varying external forces cannot generally be described through a fixed bottleneck near the transition-state barrier or dividing surface. A naive dividing surface attached to the instantaneous, but moving, barrier top also fails to be recrossing-free. We construct a moving dividing surface in phase space over a transition-state trajectory. This surface is recrossing-free for both Hamiltonian and dissipative dynamics. This is confirmed even for strongly anharmonic barriers using simulation. The power of transition-state theory is thereby applicable to chemical reactions and other activated processes even when the bottlenecks are time dependent and move across space.

Craven, Galen T.; Bartsch, Thomas; Hernandez, Rigoberto

2014-04-01

316

THE LIFETIME OF AEROSOLS IN AMBIENT AIR: CONSIDERATION OF THE EFFECTS OF SURFACTANTS AND CHEMICAL REACTIONS  

SciTech Connect

The relatively long lifetime of droplets in atmospheric haze and fog in comparison with similar droplets of pure water is attributed to the presence of a monolayer of surfactant film and to the accumulation Of soluble salts from chemical reactions. The lifetime of these droplets is a significant factor in the evaluation of the role of heterogeneous aqueous chemical reactions occurring in the troposphere. Several mechanisms of SO{sub 2} oxidation in the presence of liquid water are investigated. It is shown that soot-catalyzed oxidation of sulfur dioxide could be responsible for the high level of sulfate concentration observed in the coastal industrial areas.

Toossi, R.; Novakov, T.

1984-04-01

317

Modeling of multiphase flow with solidification and chemical reaction in materials processing  

NASA Astrophysics Data System (ADS)

Understanding of multiphase flow and related heat transfer and chemical reactions are the keys to increase the productivity and efficiency in industrial processes. The objective of this thesis is to utilize the computational approaches to investigate the multiphase flow and its application in the materials processes, especially in the following two areas: directional solidification, and pyrolysis and synthesis. In this thesis, numerical simulations will be performed for crystal growth of several III-V and II-VI compounds. The effects of Prandtl and Grashof numbers on the axial temperature profile, the solidification interface shape, and melt flow are investigated. For the material with high Prandtl and Grashof numbers, temperature field and growth interface will be significantly influenced by melt flow, resulting in the complicated temperature distribution and curved interface shape, so it will encounter tremendous difficulty using a traditional Bridgman growth system. A new design is proposed to reduce the melt convection. The geometric configuration of top cold and bottom hot in the melt will dramatically reduce the melt convection. The new design has been employed to simulate the melt flow and heat transfer in crystal growth with large Prandtl and Grashof numbers and the design parameters have been adjusted. Over 90% of commercial solar cells are made from silicon and directional solidification system is the one of the most important method to produce multi-crystalline silicon ingots due to its tolerance to feedstock impurities and lower manufacturing cost. A numerical model is developed to simulate the silicon ingot directional solidification process. Temperature distribution and solidification interface location are presented. Heat transfer and solidification analysis are performed to determine the energy efficiency of the silicon production furnace. Possible improvements are identified. The silicon growth process is controlled by adjusting heating power and moving the side insulation layer upward. It is possible to produce high quality crystal with a good combination of heating and cooling. SiC based ceramic materials fabricated by polymer pyrolysis and synthesis becomes a promising candidate for nuclear applications. To obtain high uniformity of microstructure/concentration fuel without crack at high operating temperature, it is important to understand transport phenomena in material processing at different scale levels. In our prior work, a system level model based on reactive porous media theory was developed to account for the pyrolysis process in uranium-ceramic nuclear fabrication In this thesis, a particle level mesoscopic model based on the Smoothed Particle Hydrodynamics (SPH) is developed for modeling the synthesis of filler U3O8 particles and SiC matrix. The system-level model provides the thermal boundary conditions needed in the particle level simulation. The evolution of particle concentration and structure as well as composition of composite produced will be investigated. Since the process temperature and heat flux play the important roles in material quality and uniformity, the effects of heating rate at different directions, filler particle size and distribution on uniformity and microstructure of the final product are investigated. Uncertainty issue is also discussed. For the multiphase flow with directional solidification, a system level based on FVM is established. In this model, melt convection, temperature distribution, phase change and solidification interface can be investigated. For the multiphase flow with chemical reaction, a particle level model based on SPH method is developed to describe the pyrolysis and synthesis process of uranium-ceramic nuclear fuel. Due to its mesh-free nature, SPH can easily handle the problems with multi phases and components, large deformation, chemical reactions and even solidifications. A multi-scale meso-macroscopic approach, which combine a mesoscopic model based on SPH method and macroscopic model based on FVM, FEM and FDM, can be applied to even more com

Wei, Jiuan

318

Homodyne High-Harmonic Spectroscopy: Coherent Imaging of a Unimolecular Chemical Reaction  

NASA Astrophysics Data System (ADS)

At the heart of high harmonic generation lies a combination of optical and collision physics entwined by a strong laser field. An electron, initially tunnel-ionized by the field, driven away then back in the continuum, finally recombines back to rest in its initial ground state via a radiative transition. The emitted attosecond (atto=10-18) XUV light pulse carries all the information (polarization, amplitude and phase) about the photorecombination continuum-to-ground transition dipolar field. Photorecombination is related to the time-reversed photoionization process. In this perspective, high-harmonic spectroscopy extends well-established photoelectron spectroscopy, based on charged particle detection, to a fully coherent one, based on light characterization. The main achievement presented in this thesis is to use high harmonic generation to probe femtosecond (femto=10-15) chemical dynamics for the first time. Thanks to the coherence imposed by the strong driving laser field, homodyne detection of attosecond pulses from excited molecules undergoing dynamics is achieved, the signal from unexcited molecules acting as the reference local oscillator. First, applying time-resolved high-harmonic spectroscopy to the photodissociation of a diatomic molecule, Br2 ? Br + Br, allows us to follow the break of a chemical bond occurring in a few hundreds of femtoseconds. Second, extending it to a triatomic (NO2) lets us observe both the previously unseen (but predicted) early femtosecond conical intersection dynamics followed by the late picosecond statistical photodissociation taking place in the reaction NO2 ? NO + O. Another important realization of this thesis is the development of a complementary technique to time-resolved high-harmonic spectroscopy called LAPIN, for Linked Attosecond Phase INterferometry. When combined together, time-resolved high-harmonic spectroscopy and LAPIN give access to the complex photorecombination dipole of aligned excited molecules. These achievements lay the basis for electron recollision tomographic imaging of a chemical reaction with unprecedented angstrom (1 angstrom= 0.1 nanometer) spatial resolution. Other contributions dedicated to the development of attosecond science and the generalization of high-harmonic spectroscopy as a novel, fully coherent molecular spectroscopy will also be presented in this thesis.

Beaudoin Bertrand, Julien

319

The importance of early erythematous reaction in purified protein derivative skin testing.  

PubMed

We attempted to assess the diagnostic value of the early erythematous reaction observed at the sixth hour after the application of purified protein derivative (PPD) skin testing. For this purpose, 64 children with pulmonary tuberculosis and 49 healthy age-matched controls were PPD skin tested. Our results showed that the erythematous reaction of 5 mm or greater at the sixth hour was able to detect patients with active tuberculosis with 76% sensitivity, 85% specificity, 87% positive predictivity and 73% negative predictivity. Among 113 subjects, 6 h erythematous reaction of 5 mm or greater in size had 83% sensitivity to detect the ones who subsequently developed 10 mm or greater induration reaction at 48 h. We concluded that the sixth hour early erythematous reaction is just as helpful as the 48 h induration of 10 mm or greater in detecting patients with pulmonary tuberculosis. PMID:7801357

Barlan, I B; Basaran, M M

1994-10-01

320

Isotope modeling of nitric acid formation in the atmosphere: Testing the importance of NO oxidation, heterogeneous reactions, and nighttime chemistry  

NASA Astrophysics Data System (ADS)

Atmospheric nitrate (NO3-atm = HNO3 and particulate nitrate) is the primary sink of NOx and its isotopic composition can yield important information about oxidation pathways. In particular oxygen isotope anomalies, the 17O excess (?17O values), have potential to determine the relative importance of NOx oxidation pathways (OH, NO3 or N2O5). We have developed a new isotope tracer model to test whether ?17O is a viable tracer for heterogeneous N2O5 hydrolysis and or NO3 radical reactions. The model uses the Regional Atmospheric Chemical Mechanism (RACM) and the isotope mass balance transfer mechanism we previously suggested. RACM was modified to account for heterogeneous uptake using three different parameterization schemes. We then conducted a sensitivity analysis that varied trace gas concentrations (NOx, VOC’s, biogenic organics), aerosol properties (mass, composition), and meteorology (RH, temperature, solar flux) and monitored the ?17O values in NO3-atm product. The model predicts that under most conditions, NO3-atm ?17O values are between 19 and 34‰ in agreement with observations. The model predicts that the D17O value in NO3-atm should have temperature dependence, as well as a seasonal effect. There was a small, but significant difference in predicted ?17O values based on which N2O5 hydrolysis scheme was used suggesting that ?17O analysis of NO3-atm may be a way of validating which scheme is most accurately reflecting the actual chemistry of the atmosphere. The model output results suggest that the oxidation pathway of NO3 to NO2, either by ozone or by peroxy radicals, has a pronounced impact on the resulting ?17O values and can lead to ?17O values lower than those observed in the existing data sets (8-18‰). There is a strong correlation between NO oxidation pathway and ?17O value in NO3-atm. The accuracy of the predictions of ?17O values made by the model can be tested by selecting unique environments and collecting NO3-atm then analyzing it for its ?17O composition. Such a data set may be helpful in understanding the role of nighttime and heterogeneous reactions in converting NOx into NO3-atm.

Michalski, G.; Xu, F.

2009-12-01

321

Chemical reactions in the gas phase and in condensed matter: From wavefunctions to density operators  

NASA Astrophysics Data System (ADS)

This contribution generalizes the treatment of chemical reactions in the gas phase based on the reaction channel decomposition of the wavefunction, by introducing a similar channel decomposition of the statistical density operator valid also for condensed phases such as liquid solutions and solid surfaces. Coupled equations for the channel components of the density operator are derived and a brief presentation is given of their partial Wigner transform, which leads to a general treatment for coupling quantum and classical variables. This provides a general approach for reactions involving electronically excited states in many-atom systems. It is pointed out that reactions involving coupled quantal and classical variables can be correctly described provided (a) initial conditions for trajectories are generated from quantal distributions and (b) the bundle of trajectories for the whole initial classical phase space is propagated coupled to the quantal elements of the density matrix and used in the calculation of reaction flux averages.

Micha, David A.

322

Studies in photochemical smog chemistry. I. Atmospheric chemistry of toluene. II. Analysis of chemical reaction mechanisms for photochemical smog  

SciTech Connect

This study focuses on two related topics in the gas phase organic chemistry of importance in urban air pollution. Part I describes an experimental and modeling effort aimed at developing a new explicit reaction mechanism for the atmospheric photooxidation of toluene. This mechanism is tested using experimental data from both indoor and outdoor smog chamber facilities. The predictions of the new reaction mechanism are found to be in good agreement with both sets of experimental data. Additional simulations performed with the new mechanism are used to investigate various mechanistic paths, and to gain insight into areas where the understanding is not complete. The outdoor experimental facility, which was built to provide the second set of experimental data, consists of a 65 cubic meter teflon smog chamber together with full instrumentation capable of measuring ozone, nitrogen dioxide, nitric oxide, peroxyacetyl nitrate (PAN), carbon monoxide, relative humidity, temperature, aerosol size distributions, and of course toluene and its photooxidation products. In Part II, a theoretical analysis of lumped chemical reaction mechanisms for photochemical smog is presented. Included is a description of a new counter species analysis technique which can be used to analyze any complex chemical reaction mechanism. Finally, a new lumped mechanism for photochemical smog is developed and tested against experimental data from two smog chamber facilities. Advantages of this mechanism relative to the existing lumped mechanisms are discussed.

Leone, J.A.

1985-01-01

323

Dispersion by chemical reaction of Rocky Mountain Arsenal Basin F waste soils  

Microsoft Academic Search

Many military installations have soil contamination problems that range from heavy metals to petroleum products. Rocky Mountain Arsenal (RMA) Basin F contains high concentrations of salts, heavy metals, ammonia, urea, and organics. The Dispersion by Chemical Reaction (DCR) process leads to a reduction in the mobility of the organic and inorganic constituents by first removing volatile constituents via steam stripping

J. R. Payne; G. M. Marion

1997-01-01

324

Reaction and diffusion kinetics during the initial stages of isothermal chemical vapor infiltration  

Microsoft Academic Search

This paper reports that individual bundles of ceramic fibers were infiltrated with SiC to study the reaction and diffusion kinetics during isothermal chemical vapor infiltration (CVI). More uniform infiltration was observed in samples where baffles were placed in the reactor and when HC1 was added to the inlet gases. The evolution of the microstructure was modeled using an analytical expression

Brian W. Sheldon; Theodore M. Besmann

1991-01-01

325

Hydrothermal Experiments for Understanding Chemical Reaction Diversity of Water / Rock Interaction under Sub- and Supercritical States  

NASA Astrophysics Data System (ADS)

Chemical reaction diversity was examined by hydrothermal experiments using batch and flow through type autoclaves. Dissolution experiments of granite and quartz with pure water up to 600°C and 60 MPa were carried out in order to evaluate chemical reaction under sub-and supercritical conditions. The supercritical region up to critical point for water has been inferred to be a homogeneous state, which conforms to a true liquid phase, nor true vapor phase. In terms of dissolution of granite and quartz, the supercritical state, being artificially defined as higher pressures and temperatures of the critical point, can be subdivided into two apparent phases, comprising a `liquid-like' region and a `vapor-like' region. The critical point of various kinds of geofluid, which was composed from solution in H2O-CO2-X (NaCl) system was experimentally determined by visible type autoclave, attached with transparent sapphire windows. Chemical reaction diversity, depending on location of the critical point for a given solution and apparent phase boundary with respect to chemical reaction within the supercritical state, was recognized by hydrothermal experiments.

Tsuchiya, Noriyoshi

2006-05-01

326

DEVELOPMENT AND TESTING OF A SURROGATE SPECIES CHEMICAL REACTION MECHANISM. VOLUME 2  

EPA Science Inventory

A photochemical reaction mechanism has been updated and extensively evaluated. The testing and refinement of the surrogate species mechanism was performed in order to create an improved chemical mechanism for the atmospheric simulation models that are used to develop ozone contro...

327

DEVELOPMENT AND TESTING OF A SURROGATE SPECIES CHEMICAL REACTION MECHANISM. VOLUME 1  

EPA Science Inventory

A photochemical reaction mechanism has been updated and extensively evaluated. The testing and refinement of the surrogate species mechanism was performed in order to create an improved chemical mechanism for the atmospheric simulation models that are used to develop ozone contro...

328

A simple model of mixing and chemical reaction in a turbulent shear layer  

Microsoft Academic Search

Arguments are presented to show that the concept of gradient diffusion is inapplicable to mixing in turbulent shear layers. A new model is proposed for treating molecular mixing and chemical reaction in such flows at high Reynolds number. It is based upon the experimental observations that revealed the presence of coherent structures and that showed that fluid elements from the

J. E. Broadwell; R. E. Breidenthal

1982-01-01

329

Stepwise chemical reaction strategy for highly sensitive electrochemiluminescent detection of dopamine.  

PubMed

A stepwise chemical reaction strategy based on the specific recognition of boronic acid to diol, and N-hydroxysuccinimide (NHS) ester to amine group, was designed to construct a "signal on" electrochemiluminescence (ECL) platform for highly sensitive detection of dopamine. A boronic acid-functionalized pyrene probe was synthesized and was self-assembled on the sidewalls of carbon nanotubes via ?-? stacking interactions as capture probes on a glassy carbon electrode. Meanwhile, 3,3'-dithiodipropionic acid di(N-hydroxysuccinimide ester) (DSP)-functionalized CdTe quantum dots (QDs) were designed as signal probes and characterized with transmission electron microscopy and spectroscopic techniques. Upon stepwise chemical reaction of dopamine with boronic acid and then DSP-QDs, the QDs were captured on the electrode as ECL emitters for signal readout, leading to an ultralow background signal. By using O2 as an endogenous coreactant, the "signal on" ECL method was employed to quantify the concentration of dopamine from 50 pM to 10 nM with a detection limit of 26 pM. Moreover, the stepwise chemical reaction-based biosensor showed high specificity against cerebral interference and was successfully applied in the detection of dopamine in cerebrospinal fluid samples. The stepwise chemical reaction strategy should be a new concept for the design of highly selective analytical methods for the detection of small biomolecules. PMID:23931569

Zhang, Lei; Cheng, Yan; Lei, Jianping; Liu, Yueting; Hao, Qing; Ju, Huangxian

2013-08-20

330

[Evaluation of imported Plasmodium falciparum malaria cases: the use of polymerase chain reaction in diagnosis].  

PubMed

Malaria affecting almost half of the world population continues to be an important health problem. Although domestic malaria cases have been decreasing in Turkey recently, cases caused by Plasmodium falciparum have increased due to the frequent travelling to Africa. The aims of this study were to evaluate demographic characteristics, clinical and laboratory findings in cases with falciparum malaria who attended to our clinic in 2012-2013 period, and the impact of polymerase chain reaction (PCR) for diagnosis. Nine patients evaluated were all male with a mean age of 34.3 (age range: 18-48) years, with the history of travel to Africa. Six cases did not take prophylaxis against malaria and other three cases used insufficient time. Mean duration of symptoms after return was 18.4 (range: 1-75) days, and the patients were admitted to the clinic within a mean of 5.2 (range: 1-15) days. Two patients had leucopenia, two patients had anemia, and eight patients had thrombocytopenia on admission. Alanine aminotransferase (ALT) levels in four cases and total bilirubin levels of six cases were over upper normal limits. Definitive diagnosis of cases was performed with the detection of ring and/or gametocytes forms of the parasite in Giemsa-stained peripheral blood smears. Furthermore, samples from seven patients were studied by nested PCR by using genus (Plasmodium rPLU 1 and 5) and species (rFAL 1 and 2, rVIV 1 and 2, rMAL 1 and 2, rOVA 1 and 2) specific primers. All of these seven samples yielded positive results with primers specific for P.falciparum ssrRNA. In the treatment, arthemeter/lumefantrin and doxycycline combination was used in seven patients, while intravenous artesunate and doxycycline combination was given to two patients, resulting with complete cure. Mean duration for the resolving of fever was 3.3 days, and mean duration for clearing the parasitemia from peripheral blood was 4.9 days. Initial ALT values and the duration of fever resolution (-796; p= 0.010), as well as the duration of parasitemia and initial thrombocyte counts (-797; p= 0.010) were negatively- correlated. It was concluded that, providing sufficient information on malaria and prophylaxis to people travelling to the endemic areas are crutial for protection. Moreover, in endemic areas for Crimean-Congo hemorrhagic fever, patients with fever and thrombocytopenia should be questioned in detail about the travel history, and peripheral blood smears should be examined in terms of malaria, since their clinical features are similar. Plasmodium PCR should be considered as one of the alternative diagnostic method in malaria, especially in cases with inconclusive microscopy. PMID:24237435

Demiraslan, Hayati; Erdo?an, Emrah; Türe, Zeynep; Kuk, Salih; Yazar, Süleyman; Metan, Gökhan

2013-10-01

331

Looking for chemical reaction networks exhibiting a drift along a manifold of marginally stable states.  

PubMed

I recently reported some examples of mass-action equations that have a continuous manifold of marginally stable stationary states [Brogioli, D., 2010. Marginally stable chemical systems as precursors of life. Phys. Rev. Lett. 105, 058102; Brogioli, D., 2011. Marginal stability in chemical systems and its relevance in the origin of life. Phys. Rev. E 84, 031931]. The corresponding chemical reaction networks show nonclassical effects, i.e. a violation of the mass-action equations, under the effect of the concentration fluctuations: the chemical system drifts along the marginally stable states. I proposed that this effect is potentially involved in abiogenesis. In the present paper, I analyze the mathematical properties of mass-action equations of marginally stable chemical reaction networks. The marginal stability implies that the mass-action equations obey some conservation law; I show that the mathematical properties of the conserved quantity characterize the motion along the marginally stable stationary state manifold, i.e. they allow to predict if the fluctuations give rise to a random walk or a drift under the effect of concentration fluctuations. Moreover, I show that the presence of the drift along the manifold of marginally stable stationary-states is a critical property, i.e. at least one of the reaction constants must be fine tuned in order to obtain the drift. PMID:23160143

Brogioli, Doriano

2013-02-01

332

Importance of structural and chemical heterogeneity of activated carbon surfaces for adsorption of dibenzothiophene  

SciTech Connect

The performance of various activated carbons obtained from different carbon precursors (i.e., plastic waste, coal, and wood) as adsorbents for the desulfurization of liquid hydrocarbon fuels was evaluated. To increase surface heterogeneity, the carbon surface was modified by oxidation with ammonium persulfate. The results showed the importance of activated carbon pore sizes and surface chemistry for the adsorption of dibenzothiophene (DBT) from liquid phase. Adsorption of DBT on activated carbons is governed by two types of contributions: physical and chemical interactions. The former include dispersive interactions in the microporous network of the carbons. While the volume of micropores governs the amount physisorbed, mesopores control the kinetics of the process. On the other hand, introduction of surface functional groups enhances the performance of the activated carbons as a result of specific interactions between the acidic centers of the carbon and the basic structure of DBT molecule as well as sulfur-sulfur interactions.

Ania, C.O.; Bandosz, T.J. [CUNY City College, New York, NY (United States). Dept. of Chemistry

2005-08-16

333

Progression in high school students (aged 16-18) conceptualizations about chemical reactions in solution  

NASA Astrophysics Data System (ADS)

The purpose of this study was to explore the development over time of students' understandings of the concept of chemical reaction in the context of two familiar reactions in solution. The study is based on interviews of 48 students, aged 16-18, who had been successful in their year 11 examinations and had selected to study chemistry as one of their advanced level subjects in year 12 (age 16-17) and year 13 (age 17-18). All the students were interviewed (on a one-to-one basis) twice, once in year 12 and again in year 13, allowing progression in their understanding to be studied. The results show that students made some progress in their understanding of the concept of chemical reaction, but some fundamental misconceptions remained.

Boo, Hong-Kwen; Watson, J. R.

2001-09-01

334

Use of Site-Specifically Tethered Chemical Nucleases to Study Macromolecular Reactions  

PubMed Central

During a complex macromolecular reaction multiple changes in molecular conformation and interactions with ligands may occur. X-ray crystallography may provide only a limited set of snapshots of these changes. Solution methods can augment such structural information to provide a more complete picture of a macromolecular reaction. We analyzed the changes in protein conformation and protein:nucleic acid interactions which occur during transcription initiation by using a chemical nuclease tethered to cysteines introduced site-specifically into the RNA polymerase of bacteriophage T7 (T7 RNAP). Changes in cleavage patterns as the polymerase steps through transcription reveal a series of structural transitions which mediate transcription initiation. Cleavage by tethered chemical nucleases is seen to be a powerful method for revealing the conformational dynamics of macromolecular reactions, and has certain advantages over cross-linking or energy transfer approaches.

Mukherjee, Srabani

2003-01-01

335

Modeling pore collapse and chemical reactions in shock-loaded HMX crystals  

NASA Astrophysics Data System (ADS)

The localization of deformation in shock-loaded crystals of high explosive material leads to the formation of hot spots, which, if hot enough, initiate chemical reactions. The collapse of microscopic pores contained within a crystal is one such process that localizes energy and generates hot spots. Given the difficulty of resolving the details of pore collapse in shock compression experiments, it is useful to study the problem using direct numerical simulation. In this work, we focus on simulating the shock-induced closure of a single pore in crystalline ?-HMX using a multiphysics finite element code. To address coupled thermal-mechanical-chemical responses, the model incorporates a crystal-mechanics-based description of thermoelasto-viscoplasticity, the crystal melting behavior, and transformation kinetics for a single-step decomposition reaction. The model is applied to stress wave amplitudes of up to 11 GPa to study the details of pore collapse, energy localization, and the early stages of reaction initiation.

Austin, R. A.; Barton, N. R.; Howard, W. M.; Fried, L. E.

2014-05-01

336

KINETICS: A computer program to analyze chemical reaction data. Revision 2  

SciTech Connect

KINETICS (Version 3.2) is a copyrighted, user-friendly kinetics analysis computer program designed for reactions such-as kerogen or polymer decomposition. It can fit rate parameters to chemical reaction data (rate or cumulative reacted) measured at a series of constant temperatures, constant heating rates, or arbitrary thermal histories. The program uses two models with conversion-dependent Azrhenius parameters and two models with activation energy distributions. The discrete distribution model fits an average frequency factor and relative fractions and activation energies for up to 25 parallel, fast-order reactions. The Gaussian distribution model fits a frequency factor, activation energy, Gaussian distribution parameter, and reaction order for up to 3 parallel reactions. For both distribution models, if the experiments are at a series of constant heating rates, the program uses a very fast approximate fitting procedure to determine possible initial parameter-estimates for the subsequent nonlinear regression analysis. This increases the probability that the regression analysis will properly. converge with a minimum of computer time. Once convergence is reached by the discrete model, the parameter space is further systematically searched to achieve global convergence. With the Gaussian model, the calculated rates or integrals can be convoluted with an experimental tracer signal during the nonlinear regression to account for dispersion effects often found in real chemical reaction data. KINETICS can also be used in an application mode to calculate reaction rates and integrals for previously determined Gaussian or discrete, parameters, using an arbitrary thermal history. Four additional models have been incorporated for the kinetics analysis of polymers and other materials, including some kerogens, which have a reaction-rate profile that is narrower than that for a single first-order reaction.

Braun, R.L.; Burnham, A.K.

1994-09-01

337

Chemical kinetic analysis of hydrogen-air ignition and reaction times  

NASA Technical Reports Server (NTRS)

An anaytical study of hydrogen air kinetics was performed. Calculations were made over a range of pressure from 0.2 to 4.0 atm, temperatures from 850 to 2000 K, and mixture equivalence ratios from 0.2 to 2.0. The finite rate chemistry model included 60 reactions in 20 species of the H2-O2-N2 system. The calculations also included an assessment of how small amounts of the chemicals H2O, NOx, H2O2, and O3 in the initial mixture affect ignition and reaction times, and how the variation of the third body efficiency of H2O relative of N2 in certain key reactions may affect reaction time. The results indicate that for mixture equivalence ratios between 0.5 and 1.7, ignition times are nearly constant; however, the presence of H2O and NO can have significant effects on ignition times, depending on the mixture temperature. Reaction time is dominantly influenced by pressure but is nearly independent of initial temperature, equivalence ratio, and the addition of chemicals. Effects of kinetics on reaction at supersonic combustor conditions are discussed.

Rogers, R. C.; Schexnayder, C. J., Jr.

1981-01-01

338

Chemical reactions in TEOS/ozone chemical vapor deposition[TetraEthylOrtho Silicate  

SciTech Connect

A reaction mechanism for TEOS/O{sub 3} CVD in a SVG/WJ atmospheric pressure furnace belt reactor has been developed and calibrated with experimental deposition rate data. One-dimensional simulations using this mechanism successfully reproduce the trends observed in a set of 31 experimental runs in a WJ-TEOS999 reactor. Two-dimensional simulations using this mechanism successfully reproduce the average deposition rates for 3 different experimental conditions in a WJ-1500TF reactor, although the deposition profiles predicted by the model are flatter than the experimental static prints.

HO,PAULINE

2000-02-01

339

General chemical kinetics computer program for static and flow reactions, with application to combustion and shock-tube kinetics  

NASA Technical Reports Server (NTRS)

A general chemical kinetics program is described for complex, homogeneous ideal-gas reactions in any chemical system. Its main features are flexibility and convenience in treating many different reaction conditions. The program solves numerically the differential equations describing complex reaction in either a static system or one-dimensional inviscid flow. Applications include ignition and combustion, shock wave reactions, and general reactions in a flowing or static system. An implicit numerical solution method is used which works efficiently for the extreme conditions of a very slow or a very fast reaction. The theory is described, and the computer program and users' manual are included.

Bittker, D. A.; Scullin, V. J.

1972-01-01

340

Chlorination of N-methylacetamide and amide-containing pharmaceuticals. Quantum-chemical study of the reaction mechanism.  

PubMed

Chlorination of amides is of utmost importance in biochemistry and environmental chemistry. Despite the huge body of data, the mechanism of reaction between amides and hypochlorous acid in aqueous environment remains unclear. In this work, the three different reaction pathways for chlorination of N-methylacetamide by HOCl have been considered: the one-step N-chlorination of the amide, the chlorination via O-chlorinated intermediate, and the N-chlorination of the iminol intermediate. The high-level quantum chemical G3B3 composite procedure, double-hybrid B2-PLYPD, B2K-PLYP methods, and global hybrid M06-2X and BMK methods have been employed. The calculated energy barriers have been compared to the experimental value of ?G(#)298 ? 87 kJ/mol, which corresponds to reaction rate constant k(r) ? 0.0036 M(-1) s(-1). Only the mechanism in which the iminol form of N-methylacetamide reacts with HOCl is consistent (?G(#)298 = 87.3 kJ/mol at G3B3 level) with experimental results. The analogous reaction mechanism has been calculated as the most favorable pathway in the chlorination of small-sized amides and amide-containing pharmaceuticals: carbamazepine, acetaminophen, and phenytoin. We conclude that the formation of the iminol intermediate followed by its reaction with HOCl is the general mechanism of N-chlorination for a vast array of amides. PMID:24601593

Šaki?, Davor; Šonji?, Pavica; Tandari?, Tana; Vr?ek, Valerije

2014-03-27

341

The activation strain model and molecular orbital theory: understanding and designing chemical reactions.  

PubMed

In this Tutorial Review, we make the point that a true understanding of trends in reactivity (as opposed to measuring or simply computing them) requires a causal reactivity model. To this end, we present and discuss the Activation Strain Model (ASM). The ASM establishes the desired causal relationship between reaction barriers, on one hand, and the properties of reactants and characteristics of reaction mechanisms, on the other hand. In the ASM, the potential energy surface ?E(?) along the reaction coordinate ? is decomposed into the strain ?Estrain(?) of the reactants that become increasingly deformed as the reaction proceeds, plus the interaction ?Eint(?) between these deformed reactants, i.e., ?E(?) = ?Estrain(?) + ?Eint(?). The ASM can be used in conjunction with any quantum chemical program. An analysis of the method and its application to problems in organic and organometallic chemistry illustrate the power of the ASM as a unifying concept and a tool for rational design of reactants and catalysts. PMID:24699791

Fernández, Israel; Bickelhaupt, F Matthias

2014-07-21

342

Direct observation of chemical reactions on single gold nanocrystals using surface plasmon spectroscopy.  

PubMed

Heterogeneous catalysts have been pivotal to the development of the modern chemical industry and are essential for catalysing many industrial reactions. However, reaction rates are different for every individual catalyst particle and depend upon each particle's morphology and size, crystal structure and composition. Measuring the rates of reaction on single nanocrystals will enable the role of catalyst structure to be quantified. Here, using surface plasmon spectroscopy, we have directly observed the kinetics of atomic deposition onto a single gold nanocrystal and also monitored electron injection and extraction during a redox reaction involving the oxidation of ascorbic acid on a gold nanocrystal surface. These results constitute the first direct measurement of the rates of redox catalysis on single nanocrystals. PMID:18838998

Novo, Carolina; Funston, Alison M; Mulvaney, Paul

2008-10-01

343

Method and apparatus for obtaining enhanced production rate of thermal chemical reactions  

DOEpatents

The present invention is a method and apparatus (vessel) for providing a heat transfer rate from a reaction chamber through a wall to a heat transfer chamber substantially matching a local heat transfer rate of a catalytic thermal chemical reaction. The key to the invention is a thermal distance defined on a cross sectional plane through the vessel inclusive of a heat transfer chamber, reaction chamber and a wall between the chambers. The cross sectional plane is perpendicular to a bulk flow direction of the reactant stream, and the thermal distance is a distance between a coolest position and a hottest position on the cross sectional plane. The thermal distance is of a length wherein the heat transfer rate from the reaction chamber to the heat transfer chamber substantially matches the local heat transfer rate.

Tonkovich, Anna Lee Y [Pasco, WA; Wang, Yong [Richland, WA; Wegeng, Robert S [Richland, WA; Gao, Yufei [Kennewick, WA

2003-04-01

344

SYBYL line notation (SLN): a single notation to represent chemical structures, queries, reactions, and virtual libraries.  

PubMed

SYBYL line notation (SLN) is a powerful way to represent molecular structures, reactions, libraries of structures, molecular fragments, formulations, molecular queries, and reaction queries. Nearly any chemical structure imaginable, including macromolecules, pharmaceuticals, catalysts, and even combinatorial libraries can be represented as an SLN string. The language provides a rich syntax for database queries comparable to SMARTS. It provides full Markush, R-Group, reaction, and macro atom capabilities in a single unified notation. It includes the ability to specify 3D conformations and 2D depictions. All the information necessary to recreate the structure in a modeling or drawing package is present in a single, concise string of ASCII characters. This makes SLN ideal for structure communication over global computer networks between applications sitting at remote sites. Unlike SMILES and its derivatives, SLN accomplishes this within a single unified syntax. Structures, queries, compounds, reactions, and virtual libraries can all be represented in a single notation. PMID:18998666

Homer, R Webster; Swanson, Jon; Jilek, Robert J; Hurst, Tad; Clark, Robert D

2008-12-01

345

Laboratory studies of sticking coefficients and heterogeneous reactions important in the Antarctic stratosphere  

NASA Technical Reports Server (NTRS)

Laboratory studies of sticking coefficients of H2O, HCl, Cl2, and HNO3 on ice and heterogeneous reactions of ClONO2 with ice or HCl/ice have been performed in a fast flow reactor. A quadrupole mass spectrometer with electron impact ionization was used as a detector. Measured sticking coefficients are: 0.3 (+0.7, -0.1) for H2O, 0.4 (+0.6, -0.2) for HCL, less than 0.0001 for Cl2, and 0.3 (+0.7, -0.1) for HNO3 at 200 K. The reaction probability of ClONO2 on ice was found to be 0.06 (+ or - 0.03) while HOCl was observed as a sole product in the gas phase. With HCl present in ice, the reaction probability of ClONO2 is greatly enhanced, approaching 0.27 (+0.73, -0.13) while molecular chlorine was found to be the major product in the gas phase. Another reaction product was nitric acid which remained in the solid phase. Since the polar stratospheric clouds contain ice particles or possibly HCl/ice particles on the surface, the present results should be a major factor in producing the observed springtime ozone depletion in the Antarctic stratosphere.

Leu, Ming-Taun

1988-01-01

346

CO2 in 1-butyl-3-methylimidazolium acetate. 2. NMR investigation of chemical reactions.  

PubMed

The solvation of CO(2) in 1-butyl-3-methylimidazolium acetate (Bmim Ac) has been investigated by (1)H, (13)C, and (15)N NMR spectroscopy at low CO(2) molar fraction (mf) (x(CO(2)) ca. 0.27) corresponding to the reactive regime described in part 1 of this study. It is shown that a carboxylation reaction occurs between CO(2) and Bmim Ac, leading to the formation of a non-negligible amount (~16%) of 1-butyl-3-methylimidazolium-2-carboxylate. It is also found that acetic acid molecules are produced during this reaction and tend to form with elapsed time stable cyclic dimers existing in pure acid. A further series of experiments has been dedicated to characterize the influence of water traces on the carboxylation reaction. It is found that water, even at high ratio (0.15 mf), does not hamper the formation of the carboxylate species but lead to the formation of byproduct involving CO(2). The evolution with temperature of the resonance lines associated with the products of the reactions confirms that they have a different origin. The main byproduct has been assigned to bicarbonate. All these results confirm the existence of a reactive regime in the CO(2)-Bmim Ac system but different from that reported in the literature on the formation of a reversible molecular complex possibly accompanied by a minor chemical reaction. Finally, the reactive scheme interpreting the carboxylation reaction and the formation of acetic acid proposed in the literature is discussed. We found that the triggering of the carboxylation reaction is necessarily connected with the introduction of carbon dioxide in the IL. We argue that a more refined scheme is still needed to understand in details the different steps of the chemical reaction in the dense phase. PMID:22533570

Besnard, Marcel; Cabaço, M Isabel; Vaca Chávez, Fabián; Pinaud, Noël; Sebastião, Pedro J; Coutinho, João A P; Mascetti, Joëlle; Danten, Yann

2012-05-24

347

The role of products and a reverse reaction in analysis of nonequilibrium effects in a bimolecular chemical reaction in a dilute gas  

NASA Astrophysics Data System (ADS)

A thermally activated bimolecular reaction in a dilute gas is analyzed. The Shizgal-Karplus perturbation method of solution of the Boltzmann equation for reactions A+A?B+B and A+A?B+B is used to obtain the analytical expressions for the nonequilibrium temperatures of reactants and for the rate of chemical reaction. For both the reactions the temperatures of components obtained within one and two Sonines approximations depend on concentration of products. The rate constant of chemical reaction depends on concentration of products for the first reaction only and for the second reaction is constant. The analytical results for the temperature of the reactant A are compared with those obtained from the Monte Carlo computer simulations with use of the Bird method. It is shown that all analytical expressions simplify to those previously discussed if the concentration of products is negligible.

Cukrowski, A. S.

2000-01-01

348

Mass Transfer from Gas Bubbles to Impinging Flow of Biological Fluids with Chemical Reaction  

PubMed Central

The rates of mass transfer from a gas bubble to an impinging flow of a biological fluid such as whole blood and plasma are investigated analytically and experimentally. Gases commonly found dissolved in body fluids are included. Consideration is given to the effects of the chemical reaction between the dissolved gas and the liquid on the rate of mass transfer. Through the application of boundary layer theory the over-all transfer is found to be Sh/(Re)1/2 = 0.845 Sc1/3 in the absence of chemical reaction, and Sh/(Re) 1/2 = F? (0) in the presence of chemical reaction, where Sh, Re, and Sc are the Sherwood, Reynolds, and Schmidt numbers, respectively, and F? (0) is a function of Sc and the dimensionless reaction rate constant. Analytical results are also obtained for the bubble lifetime and the bubble radius-time history. These results, which are not incompatible with experimental results, can be applied to predict the dissolution of the entrapped gas emboli in the circulatory system of the human body.

Yang, Wen-Jei; Echigo, R.; Wotton, D. R.; Ou, J. W.; Hwang, J. B.

1972-01-01

349

Solar photo-thermal catalytic reactions to produce high value chemicals  

SciTech Connect

This report presents a summary of the research work accomplished to date on the utilization of solar photo-thermal energy to convert low cost chemical feedstocks into high $-value chemical products. The rationale is that the solar IR-VIS-UV spectrum is unique, supplying endothermic reaction energy as well as VIS-UV for photochemical activation. Chemical market analysis and product price distribution focused attention on speciality chemicals with prices >$1.00/lb, and a synthesis sequence of n-paraffins to aromatics to partial oxidized products. The experimental work has demonstrated that enhanced reaction effects result from VIS-UV irradiation of catalytically active V2O5/SiO2. Experiments of the past year have been on dehydrogenation and dehydrocyclization of n-paraffins to olefins and aromatics with preference for the latter. Recent results using n-hexane produced 95% conversion with 56% benzene; it is speculated that aromatic yield should reach {approximately}70% by further optimization. Pilot- and commercial-scale reactor configurations have been examined; the odds-on-favorite being a shallow fluid-bed of catalyst with incident radiation from the top. Sequencing for maximum cost effectiveness would be day-time endothermic followed by night-time exothermic reactions to produce the products.

Prengle, H.W. Jr.; Wentworth, W.E. [Houston Univ., TX (United States)] [Houston Univ., TX (United States)

1992-04-01

350

A coupled mechanical and chemical damage model for concrete affected by alkali–silica reaction  

SciTech Connect

To model the complex degradation phenomena occurring in concrete affected by alkali–silica reaction (ASR), we formulate a poro-mechanical model with two isotropic internal variables: the chemical and the mechanical damage. The chemical damage, related to the evolution of the reaction, is caused by the pressure generated by the expanding ASR gel on the solid concrete skeleton. The mechanical damage describes the strength and stiffness degradation induced by the external loads. As suggested by experimental results, degradation due to ASR is considered to be localized around reactive sites. The effect of the degree of saturation and of the temperature on the reaction development is also modeled. The chemical damage evolution is calibrated using the value of the gel pressure estimated by applying the electrical diffuse double-layer theory to experimental values of the surface charge density in ASR gel specimens reported in the literature. The chemo-damage model is first validated by simulating expansion tests on reactive specimens and beams; the coupled chemo-mechanical damage model is then employed to simulate compression and flexure tests results also taken from the literature. -- Highlights: •Concrete degradation due to ASR in variable environmental conditions is modeled. •Two isotropic internal variables – chemical and mechanical damage – are introduced. •The value of the swelling pressure is estimated by the diffuse double layer theory. •A simplified scheme is proposed to relate macro- and microscopic properties. •The chemo-mechanical damage model is validated by simulating tests in literature.

Pignatelli, Rossella, E-mail: rossellapignatelli@gmail.com [Department of Civil and Environmental Engineering, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano (Italy) [Department of Civil and Environmental Engineering, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano (Italy); Lombardi Ingegneria S.r.l., Via Giotto 36, 20145 Milano (Italy); Comi, Claudia, E-mail: comi@stru.polimi.it [Department of Civil and Environmental Engineering, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano (Italy)] [Department of Civil and Environmental Engineering, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano (Italy); Monteiro, Paulo J.M., E-mail: monteiro@ce.berkeley.edu [Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720 (United States)

2013-11-15

351

The swelling behavior of ionic polymers in the presence of diffusion and chemical reactions  

NASA Astrophysics Data System (ADS)

During the past few decades, ionic polymers have attracted much attention due to their unique properties, i.e., a stimulus-sensitive swelling behavior. The use of ionic polymers in biotechnology and medicine is increasingly attractive for applications such as controlled drug delivery, biomimetic actuators, and chemical valves. The ability to model and predict the swelling behavior of polymers and mass flux with respect to external environmental conditions is very important in such applications. The focus of this dissertation is on developing a continuum model for the behavior of a swollen solid and diffusion of a fluid through it in the presence of chemical reactions. To model diffusion through an ionic/nonionic polymer in the presence of mechanical deformation, we develop a variational procedure based on a thermodynamic framework for a swollen solid subject to constraints. This thermodynamic framework was mainly developed by Rajagopal and Srinivasa as a general thermodynamic framework for inelastic behavior of materials. First, a model for the mechanical behavior of a rubber-like solid in the presence of mass diffusion is developed using the variational procedure. By assuming a specific form for the Helmholtz potential function, we obtain an equilibrium equation that is identical to the equation suggested by Flory, Huggins, and Rehner. For the non-equilibrium problem, we assume a specific form for the rate of dissipation function and obtain the relation between pressure difference and mass flux. The numerical results show very good agreement with experimental data for the diffusion of organic solvents through a rubber slab. Hysteresis response of the swollen solid due to time-dependent loads is also simulated by the model. The variational procedure is also applied to pH-sensitive ionic polymers, which not only yields the equations for the deformation of a swollen solid and mass flux, but also the equations for chemical equilibrium. As a specific example, an anionic polymer containing carboxylic acid in a solution of NaOH is investigated and its behavior in solutions of different pH and surrounding pressures is predicted. The numerical results show good agreement with the experimental data.

Baek, Seungik

352

A bond-topological approach to theoretical mineralogy: crystal structure, chemical composition and chemical reactions  

NASA Astrophysics Data System (ADS)

Here, I describe a theoretical approach to the structure and chemical composition of minerals based on their bond topology. This approach allows consideration of many aspects of minerals and mineral behaviour that cannot be addressed by current theoretical methods. It consists of combining the bond topology of the structure with aspects of graph theory and bond-valence theory (both long range and short range), and using the moments approach to the electronic energy density-of-states to interpret topological aspects of crystal structures. The structure hierarchy hypothesis states that higher bond-valence polyhedra polymerize to form the (usually anionic) structural unit, the excess charge of which is balanced by the interstitial complex (usually consisting of large low-valence cations and (H2O) groups). This hypothesis may be justified within the framework of bond topology and bond-valence theory, and may be used to hierarchically classify oxysalt minerals. It is the weak interaction between the structural unit and the interstitial complex that controls the stability of the structural arrangement. The principle of correspondence of Lewis acidity-basicity states that stable structures will form when the Lewis-acid strength of the interstitial complex closely matches the Lewis-base strength of the structural unit, and allows us to examine the factors that control the chemical composition and aspects of the structural arrangements of minerals. It also provides a connection between a structure, the speciation of its constituents in aqueous solution and its mechanism of crystallization. The moments approach to the electronic energy density-of-states provides a link between the bond topology of a structure and its thermodynamic properties, as indicated by correlations between average anion coordination number and reduced enthalpy of formation from the oxides for [6]Mg{/m [4]}Si n O( m+2 n) and MgSO4(H2O) n .

Hawthorne, Frank C.

2012-11-01

353

Inducing All Steps of a Chemical Reaction with the Scanning Tunneling Microscope Tip: Towards Single Molecule Engineering  

Microsoft Academic Search

All elementary steps of a chemical reaction have been successfully induced on individual molecules with a scanning tunneling microscope (STM) in a controlled step-by-step manner utilizing a variety of manipulation techniques. The reaction steps involve the separation of iodine from iodobenzene by using tunneling electrons, bringing together two resultant phenyls mechanically by lateral manipulation and, finally, their chemical association to

Saw-Wai Hla; Ludwig Bartels; Gerhard Meyer; Karl-Heinz Rieder

2000-01-01

354

A METHOD FOR CALCULATING THE EFFECTIVE PERMITTIVITY OF A MIXTURE SOLUTION DURING A CHEMICAL REACTION BY EXPERIMENTAL RESULTS  

Microsoft Academic Search

Usually, an effective permittivity can be used to describe the molecular polarization of a mixture during a chemical reaction and consequently be used to calculate the transmission and absorption of microwave in the mixture. In this paper, we propose a method to calculate the effective permittivity of a mixture solution during a chemical reaction by means of the experimental results.

Kama Huang; Xiaoqing Yang

2008-01-01

355

Modeling the effects of chemical exposure on avian seasonal productivity: Importance of differences in breeding strategies  

EPA Science Inventory

Agencies that regulate the use of chemicals are increasingly interested in understanding the magnitude of effects of those chemicals on wildlife populations. While laboratory toxicity tests provide insights into the types of effects caused by chemical exposure, they do not alway...

356

Desalination of Thermal Water from Ixtapan De La Sal, Mex. By Means of Chemical Reactions, Ultrasound and Flotation Cell  

NASA Astrophysics Data System (ADS)

It is an excellent process because, the sodium chloride was converted into AgCl and calcium, sodium sulfate. The silver can be recovery from AgCl and from the final solution. Only it is necessary to design new tests in order to improve the time duration process and chemical reactive dose. Because for the equipment used, the process can be scaled to a largest seawater desalination. The bactericidal effect of the process and ultrasound is very important because the final water could be drinkable without harmful bacteria. Only spores and fungus remained 0.35% using ultrasound. The same process could be good for the seawater desalination because the chemical reactions are the same.

Abrego, Josef.

357

Protein kinase C plays an important role in the human zona pellucida- induced acrosome reaction  

Microsoft Academic Search

To investigate the involvement of protein kinases in signal transduction in the human zona pellucida (ZP)- induced acrosome reaction (AR), the effects of protein kinase (PK) activators, dibutyryl cAMP (PKA) and cGMP (PKG), phorbol 12-myristate 13-acetate (PMA, PKC), and the PKC inhibitor, staurosporine were studied. Sperm samples were obtained from normozoospermic men with normal sperm-ZP binding. Oocytes were obtained from

D. Y. Liu; H. W. G. Baker

1997-01-01

358

Thermal decomposition reactions of HMX and RDX and their importance in predicting cookoff hazards  

Microsoft Academic Search

Recent work on the decomposition of hexahydro-1,3,5-trinitro-s-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), utilizing simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS), has shown that they undergo significant decomposition in both the solid and liquid phases. Previous studies have determined the mechanisms of several reaction pathways that control the decomposition of these materials. This paper presents a summary of this previous work and

R. Jr. Behrens; S. Bulusu

1994-01-01

359

Reactions of important OVOCs with hydrogen peroxide and ozone in the tropospheric aqueous phase  

NASA Astrophysics Data System (ADS)

Besides research on the microphysics of cloud droplets and similar aqueous systems in the troposphere, the chemistry of volatile organic compounds (VOCs) from anthropogenic and biogenic sources cannot be neglected for the understanding of tropospheric processes such as the organic particle mass formation. Emissions of biogenic volatile organic compounds (BVOCs) can exceed those of VOCs from anthropogenic sources by a factor of 10[1]. Oxidation products of BVOCs like glyoxal, methylglyoxal, glycolate, glyoxylate and pyruvate, glycolaldehyde, and the unsaturated compounds methacrolein and methyl vinyl ketone are known precursors for less volatile organic substances found in secondary organic aerosols[2,3]. Yet, the main decomposition of these substances is believed to occur via radical reactions. However, Tilgner and Herrmann[2] showed evidence that the turnovers by non-radical reactions with H2O2 or ozone and some non-oxidative organic accretion reactions may even exceed those from the most reactive species in the lower troposphere, the hydroxyl radical OH. This work investigated the reactivities of the atmospheric relevant oxidation products including pyruvic acid and glyoxylic acid towards O3 and H2O2 in the aqueous phase. Furthermore, pH effects were studied by measuring the kinetics of both the protonated and deprotonated forms. The measurements were performed using a UV/VIS-spectrometer (conventional and in addition a Stopped Flow technique) and capillary electrophoresis. In some cases the results indicate higher turnovers of H2O2 and ozone reactions compared to interactions with atmospheric radicals. The experimental data obtained will be presented and their implications for atmospheric multiphase chemistry are discussed. [1] Guenther et al., 1995, Journal of Geophysical Research - Atmosphere, 100(D5), 8873-8892. [2] Tilgner and Herrmann, 2010, Atmospheric Environment, 44, 5415-5422. [3] van Pinxteren et al., 2005, Atmospheric Environment, 39, 4305-4320.

Schöne, Luisa; Weller, Christian; Herrmann, Hartmut

2013-04-01

360

Simulations of isoprene: Ozone reactions for a general circulation/chemical transport model  

NASA Technical Reports Server (NTRS)

A parameterized reaction mechanism has been created to examine the interactions between isoprene and other tropospheric gas-phase chemicals. Tests of the parameterization have shown that its results match those of a more complex reaction set to a high degree of accuracy. Comparisons between test runs have shown that the presence of isoprene at the start of a six day interval can enhance later ozone concentrations by as much as twenty-nine percent. The test cases used no input fluxes beyond the initial time, implying that a single input of a biogenic hydrocarbon to an airmass can alter its ozone chemistry over a time scale on the order of a week.

Makar, P. A.; Mcconnell, J. C.

1994-01-01

361

Influence of Alumina Reaction Tube Impurities on the Oxidation of Chemically-Vapor-Deposited Silicon Carbide  

NASA Technical Reports Server (NTRS)

Pure coupons of chemically vapor deposited (CVD) SiC were oxidized for 100 h in dry flowing oxygen at 1300 C. The oxidation kinetics were monitored using thermogravimetry (TGA). The experiments were first performed using high-purity alumina reaction tubes. The experiments were then repeated using fused quartz reaction tubes. Differences in oxidation kinetics, scale composition, and scale morphology were observed. These differences were attributed to impurities in the alumina tubes. Investigators interested in high-temperature oxidation of silica formers should be aware that high-purity alumina can have significant effects on experiment results.

Opila, Elizabeth

1995-01-01

362

Predicting the progress of diffusively limited chemical reactions in the presence of chaotic advection.  

PubMed

The effects of chaotic advection and diffusion on fast chemical reactions in two-dimensional fluid flows are investigated using experimentally measured stretching fields and fluorescent monitoring of the local concentration. Flow symmetry, Reynolds number, and mean path length affect the spatial distribution and time dependence of the reaction product. A single parameter lambdaN , where lambda is the mean Lyapunov exponent N and is the number of mixing cycles, can be used to predict the time-dependent total product for flows having different dynamical features. PMID:16486585

Arratia, P E; Gollub, J P

2006-01-20

363

Is hydrogen abstraction an important pathway in the reaction of alkenes with the OH radical?  

PubMed

In this paper, we report the first temperature dependent kinetic measurements of the reaction of C(> or =5) 1-alkenes with OH radicals and also report the first room temperature measurements for 1-alkenes between C(8-11). The rate coefficients at room temperature and between 75 and 100 Torr for the reaction of OH with 1-pentene, 1-octene, 1-nonene, 1-decene and 1-undecene are (2.74 +/- 0.38) x 10(-11), (3.62 +/- 0.68) x 10(-11), (4.20 +/- 0.41) x 10(-11), (7.00 +/- 0.96) x 10(-11) and (12.44 +/- 1.62) x 10(-11) cm(3) molecule(-1) s(-1), respectively, at 298 K. No pressure dependence is observed, suggesting that these reactions are at their high pressure limit. Inspection of the variation of rate coefficient with temperature over the temperature range 262-300 K shows a weak negative temperature dependence, suggesting that adduct formation is taking place, however, a curved Arrhenius plot is clearly seen for 1-decene and theoretical analysis suggests that abstraction by OH is a non-negligible channel. An estimation of products formed from OH oxidation suggests that abstraction will lead to products that have a lower photochemical ozone creation potential than those formed via addition. PMID:17687482

McGillen, Max R; Percival, Carl J; Shallcross, Dudley E; Harvey, Jeremy N

2007-08-21

364

A Method for Incorporating Chemical Reactions into Multiphase Flow Models for CO2 Injection  

NASA Astrophysics Data System (ADS)

CO2 injection in deep saline aquifers involves many different processes, including multiphase flow, heat and mass transport, rock deformation and minerals precipitation and dissolution. All these processes are coupled. Therefore, their modeling is difficult and requires complex computer codes to describe and assess them numerically. We developed a method for incorporating chemical reactions into multiphase flow codes. It is limited to chemical systems that can be calculated as a function of the state variables of the multiphase flow model (e.g. liquid pressure, gas pressure, temperature). It consists of calculating the chemical composition of this system as a function of these state variables by means of chemical speciation codes and redefining the components of the multiphase flow code (e.g., water, CO2). We applied this method to incorporate chemical reactions of a H2O-CO2-NaCl-CaCO3 system into CodeBright, a code that can handle multiphase flow, heat transport, mass transport and deformation. We used this code to model CO2 injection into a saline aquifer containing calcite. The model could simulate well the interaction between the development of the CO2 bubble, dissolution of CO2 into the brine, calcite dissolution and density dependent flow.

Saaltink, M. W.; Vilarrasa, V.; De Gaspari, F.; Silva, O.; Carrera, J.

2012-04-01

365

Note on the stochastic theory of a self-catalytic chemical reaction. II  

NASA Astrophysics Data System (ADS)

The general results of article I on the stochastic representation of the macroscopic stationary state of a self-catalytic chemical system are applied to a step-by-step chemical reaction. The relaxation times to the quasi-stationary state and to the final stationary state are computed by evaluating the first two non-trivial eigenvalues of the transition matrix. The previous results of Oppenheim, Shuler and Weiss are confirmed, precised and extended. The critical and subcritical cases are treated by the same method.

Dambrine, S.; Moreau, M.

1981-04-01

366

Numerical simulation of the interaction of transport, diffusion and chemical reactions in an urban plume  

NASA Technical Reports Server (NTRS)

A model system is presented that takes into account the main physical and chemical processes on the regional scale here in an area of 100x100 sq km. The horizontal gridsize used is 2x2 sq km. For a case study, it is demonstrated how the model system can be used to separate the contributions of the processes advection, turbulent diffusion, and chemical reactions to the diurnal cycle of ozone. In this way, typical features which are visible in observations and are reproduced by the numerical simulations can be interpreted.

Vogel, Bernhard; Vogel, Heike; Fiedler, Franz

1994-01-01

367

Investigation of shock-induced and shock-assisted chemical reactions in molybdenum-silicon powder mixtures  

NASA Astrophysics Data System (ADS)

In this research, chemical reactions occurring in molybdenum and silicon powder mixtures under "shock-induced" (those occurring during the high-pressure shock state) and "shock-assisted" (those occurring subsequent to the shock event, but due to bulk temperature increases) conditions were investigated. Differences in the densities and yield strengths of the two constituents, in addition to the large heat of reaction associated with molybdenum disilicide (MoSi2) formation can lead to shock-induced as well as shock-assisted reactions, which make this an ideal system to delineate the kinetics and mechanisms of reactions occurring in shock-compressed powder mixtures. Shock recovery experiments performed on Mo + 2 Si powder mixtures employing cylindrical implosion geometry showed thermally initiated reactions. A mixed phase eutectic type microstructure of MoSi2 and Mo 5Si3, resulting from reaction occurring due to melting of both reactants, was observed in axial regions of the cylindrical compacts. In regions surrounding the mach stem, melting of only silicon and reaction occurring via dissolution and re-precipitation forming MoSi2 spherules surrounding molybdenum particles in a melted and solidified silicon matrix was observed. The planar pressure shock recovery geometry showed a single phase MoSi2, microstructure formed due to a solid-state pressure-induced reaction process. The time-resolved instrumented experiments were performed using a single stage gas gun in the velocity range of 500 m/s to 1 km/s, and employed poly-vinyl di-flouride (PVDF) stress gauges placed at the front and rear surfaces of the powder to determine the crush strength, densification history, and reaction initiation threshold conditions. Time-resolved experiments performed on ˜58% dense Mo + 2 Si powder mixtures at input stresses less than 4 GPa, showed characteristics of powder densification and dispersed propagated wave stress profiles with rise time >˜40 nanoseconds. At input stress between 4--6 GPa, the powder mixtures showed a sharp rise time (<˜10 ns) of propagated wave profile and an expanded state of products revealing conclusive evidence of shock-induced reaction. At input stresses greater than 6 GPa, the powder mixtures showed a lower rise time and transition to a low-compressibility (melt) state indicating lack of shock-induced reaction. The results of this work show that (a) premature formation of a melt phase restricts mixing between reactants and inhibits "shock-induced" reaction initiation, although "shock-assisted" reactions can still occur in time scales of thermal equilibrium, and (b) the crush strength of powder mixtures is the most important parameter that controls initiation of a "shock-induced" reaction. Reaction synthesis experiments conducted on 55--95% dense Mo + 2 Si powder mixture compacts under an applied electric field showed that SHS reactions that would have normally become extinguished without the application of electric field, were observed to be self-sustained. Under such conditions, the reaction kinetics were observed to be enhanced and the reaction products showed a highly refined microstructure.

Vandersall, Kevin Stewart

1999-10-01

368

The hydroperoxyl radical in atmospheric chemical dynamics - Reaction with carbon monoxide.  

NASA Technical Reports Server (NTRS)

Discussion of laboratory measurements which indicate that the reaction of the thermalized HO(2) radical with CO is exceedingly slow and that this reaction should not, therefore, be of any significance in atmospheric chemistry. The large discrepancy between the new results and data obtained earlier by Westenberg and de Haas (1972) is explained in terms of the reacting hydroperoxyl radical being in a non-Boltzmann distribution in the former study. It appears that the most important reactions of thermalized HO(2) in the atmosphere are those involving the trace gases of NO and sulfur dioxide.

Davis, D. D.; Payne, W. A.; Stief, L. J.

1973-01-01

369

Physical and chemical properties of some imported woods and their degradation by termites.  

PubMed

The influence of physical and chemical properties of 20 species of imported wood on degradation of the wood by termites under field conditions was studied. The wood species studied were: Sycamore maple, Acer pseudoplatanus L. (Sapindales: Sapindaceae) (from two countries), Camphor, Dryobalanops aromatic C.F.Gaertner (Malvales: Dipterocarpaceae), Beech, Fagus grandifolia Ehrhart (Fagales: Fagaceae), F. sylvatica L. (from two countries), Oak, Quercus robur L., Ash, Fraxinus angustifolia Vahl (Lamiales: Oleaceae), F. excelsior L., Padauk, Pterocarpus soyauxii Taubert (Fabales: Fabaceae), (from two countries), Jamba, Xylia dolabrifiormis Roxburgh, Shorea laevis Ridley (Malvales: Dipterocarpaceae), S. macoptera Dyer, S. robusta Roth, Teak, Tectona grandis L.f. (Lamiales: Lamiaceae) (from five countries), and rubber tree, Hevea brasiliensis Müller Argoviensis (Malpighiales: Euphorbiaceae) from India. The termites present were: Odontotermes horni (Wasmann) (Isoptera: Termitidae), O. feae, O. wallonensis, and O. obeus (Rambur). A significant conelation was found between density, cellulose, lignin, and total phenolic contents of the wood and degradation by termites. The higher the density of the wood, the lower the degradation. Similarly, higher amount of lignin and total phenolic contents ensured higher resistance, whereas cellulose drives the termites towards the wood. PMID:23906349

Shanbhag, Rashmi R; Sundararaj, R

2013-01-01

370

Physical and Chemical Properties of Some Imported Woods and their Degradation by Termites  

PubMed Central

The influence of physical and chemical properties of 20 species of imported wood on degradation of the wood by termites under field conditions was studied. The wood species studied were: Sycamore maple, Acer pseudoplatanus L. (Sapindales: Sapindaceae) (from two countries), Camphor, Dryobalanops aromatic C.F.Gaertner (Malvales: Dipterocarpaceae), Beech, Fagus grandifolia Ehrhart (Fagales: Fagaceae), F. sylvatica L. (from two countries), Oak, Quercus robur L., Ash, Fraxinus angustifolia Vahl (Lamiales: Oleaceae), F. excelsior L., Padauk, Pterocarpus soyauxii Taubert (Fabales: Fabaceae), (from two countries), Jamba, Xylia dolabrifiormis Roxburgh, Shorea laevis Ridley (Malvales: Dipterocarpaceae), S. macoptera Dyer, S. robusta Roth, Teak, Tectona grandis L.f. (Lamiales: Lamiaceae) (from five countries), and rubber tree, Hevea brasiliensis Müller Argoviensis (Malpighiales: Euphorbiaceae) from India. The termites present were: Odontotermes horni (Wasmann) (Isoptera: Termitidae), O. feae, O. wallonensis, and O. obeus (Rambur). A significant conelation was found between density, cellulose, lignin, and total phenolic contents of the wood and degradation by termites. The higher the density of the wood, the lower the degradation. Similarly, higher amount of lignin and total phenolic contents ensured higher resistance, whereas cellulose drives the termites towards the wood.

Shanbhag, Rashmi R.; Sundararaj, R.

2013-01-01

371

The role of gas-phase reactions in modeling of the forced-flow chemical vapor infiltration process  

Microsoft Academic Search

An analytical model is presented, which includes the effects of both gas-phase and surface reactions, and the pressure changes due to the chemical reactions in the forced-flow chemical vapor infiltration (FCVI) process. For the FCVI process controlled by the gas-phase reactions, improvements of the process by using the forced-flow are limited. However, for the FCVI process controlled by the surface

Ching Yi Tsai; J. N. Reddy; Seshu B. Desu; Chien C. Chiu

1993-01-01

372

The Q-K model for gas-phase chemical reaction rates  

NASA Astrophysics Data System (ADS)

The quantum-kinetic, or Q-K, model is based on the quantum vibration model that is employed in the computation of gas flows at the molecular level by the direct simulation Monte Carlo (DSMC) method. The Q-K procedure for dissociation is physically realistic within the context of the vibration model in that the reaction occurs upon the selection of the vibrational level that corresponds to dissociation. An analogous, but entirely phenomenological, procedure has been presented for endothermic exchange and chain reactions. These procedures for the endothermic reactions have been well validated, but the existing procedures for the corresponding exothermic reactions have proved to be problematic. This paper presents new procedures for the exothermic reactions that are computationally efficient and provide a near exact match with the equilibrium constant of statistical mechanics. The Q-K model does not depend on the availability of continuum rate coefficients. Instead, the simplicity of the new DSMC procedures allows analytical expressions to be written down for the corresponding rate coefficients in an equilibrium gas. These are used to validate the Q-K model for reactions in high temperature air and in hydrogen-oxygen combustion. The development of the Q-K model has been driven by the need for efficient reaction procedures in DSMC applications that often involve the computation of billions of simulated collisions. It is not intended to compete with the modern theories for gas-phase chemical reactions that employ more accurate physical representations of real reactions. At the same time, the degree of validation of the model is such that the analytical expressions for the rate coefficients that correspond to the model should be useful in their own right.

Bird, G. A.

2011-10-01

373

How green is a chemical reaction? Application of LCA to green chemistry.  

PubMed

In the present work Life Cycle Assessment (LCA) is used in order to evaluate a chemical reaction from an environmental point of view. The objective is to assess the usefulness of this methodology as an environmental tool to be applied to green chemistry. As an example, two routes of obtaining maleic anhydride are compared using LCA, to ascertain which one is the most environmentally friendly. From the results obtained in this work it can be concluded that LCA seems to be a valuable tool for the environmental assessment of a chemical reaction, because it takes into account all the life cycle stages of the process and discusses the impact of the environmental burdens inventoried according to a diversity of impact categories. PMID:12521184

Domènech, Xavier; Ayllón, José A; Peral, José; Rieradevall, Joan

2002-12-15

374

Tabletop imaging of structural evolutions in chemical reactions demonstrated for the acetylene cation.  

PubMed

The introduction of femto-chemistry has made it a primary goal to follow the nuclear and electronic evolution of a molecule in time and space as it undergoes a chemical reaction. Using Coulomb Explosion Imaging, we have shot the first high-resolution molecular movie of a to and fro isomerization process in the acetylene cation. So far, this kind of phenomenon could only be observed using vacuum ultraviolet light from a free-electron laser. Here we show that 266?nm ultrashort laser pulses are capable of initiating rich dynamics through multiphoton ionization. With our generally applicable tabletop approach that can be used for other small organic molecules, we have investigated two basic chemical reactions simultaneously: proton migration and C=C bond breaking, triggered by multiphoton ionization. The experimental results are in excellent agreement with the timescales and relaxation pathways predicted by new and quantitative ab initio trajectory simulations. PMID:25034613

Ibrahim, Heide; Wales, Benji; Beaulieu, Samuel; Schmidt, Bruno E; Thiré, Nicolas; Fowe, Emmanuel P; Bisson, Eric; Hebeisen, Christoph T; Wanie, Vincent; Giguére, Mathieu; Kieffer, Jean-Claude; Spanner, Michael; Bandrauk, André D; Sanderson, Joseph; Schuurman, Michael S; Légaré, François

2014-01-01

375

Chemical reactions between aluminum and fly ash during synthesis and reheating of Al-fly ash composite  

Microsoft Academic Search

Thermodynamic analysis indicates that there is the possibility of chemical reactions between aluminum melt and cenosphere\\u000a fly ash particles. These particles contain alumina, silica, and iron oxide, which, during solidification processing of aluminum-fly\\u000a ash composites or during holding of such composites at temperatures above the melting temperature of aluminum, are likely\\u000a to undergo chemical reduction. These chemical reactions between the

R. O. Guo; P. K. Rohatgi

1998-01-01

376

Thermal decomposition reactions of HMX and RDX and their importance in predicting cookoff hazards  

SciTech Connect

Recent work on the decomposition of hexahydro-1,3,5-trinitro-s-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), utilizing simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS), has shown that they undergo significant decomposition in both the solid and liquid phases. Previous studies have determined the mechanisms of several reaction pathways that control the decomposition of these materials. This paper presents a summary of this previous work and how the results can be applied to developing models to predict the response of energetic materials in cookoff situations.

Behrens, R. Jr. [Sandia National Labs., Livermore, CA (United States); Bulusu, S. [Army Armament Research, Development, and Engineering Center, Dover, NJ (United States)

1994-11-01

377

Kinetics of aqueous phase reactions of the SO sub 4 sup minus radical with potential importance in cloud chemistry  

SciTech Connect

A laser flash photolysis/long path absorption technique has been employed to study at 298 K the kinetics of aqueous phase reactions of the SO{sub 4}{sup {minus}} radical with a number of species commonly found in cloud water. Much lower radical concentrations were employed than in all previous direct studies of SO{sub 4}{sup {minus}} reaction kinetics. In the zero ionic strength limit, rate coefficients for SO{sub 4}{sup {minus}} reaction with the anions HSO{sub 3}{sup {minus}}, Cl{sup {minus}}, NO{sub 2}{sup {minus}}, HCOO{sup {minus}}, and CH{sub 3}COO{sup {minus}} are found to be 7.5 {times} 10{sup 8}, 2.6 {times} 10{sup 8}, 9.8 {times} 10{sup 8}, 1.1 {times} 10{sup 8}, and 3.7 {times} 10{sup 6} M{sup {minus}1}s{sup {minus}1}, respectively. Rate coefficients for SO{sub 4}{sup {minus}} reactions with the neutral species H{sub 2}O{sub 2}, CH{sub 3}OH, HCOOH, and CH{sub 3}COOH are found to be 1.2 {times} 10{sup 7}, 8.8 {times} 10{sup 6}, 4.6 {times} 10{sup 5}, and 1.4 {times} 10{sup 4} M{sup {minus}1}s{sup {minus}1}, respectively. For many of the reactions studied, agreement with previously reported rate data is poor. Of particular importance for cloud chemistry is the fact that the SO{sub 4}{sup {minus}} + HSO{sub 3}{sup {minus}} reaction proceeds more slowly than previously believed, while the SO{sub 4}{sup {minus}} + Cl{sup {minus}} reaction is somewhat faster than previously thought. Incorporation of these results into cloud chemistry models should reduce the calculated efficiency of free radical chain reactions as an oxidation mechanism for S IV.

Wine, P.H.; Tang, Y.; Thorn, R.P.; Wells, J.R.; Davis, D.D. (Georgia Inst. of Tech., Atlanta (USA))

1989-01-20

378

Thermodynamics of phase transitions and chemical reactions in the presence of quasistatic electromagnetic fields  

Microsoft Academic Search

Phase transitions and chemical reactions in the presence of electromagnetic fields are considered. The field-dependent differential of the internal energy is described using four basic independent terms. These terms stand for the differentials of heat T-hatdS, mechanical work -P-hatdV, work associated with mass transfer zeta-hatdN, and work delivered by the current source VH.dB. In this context, the field-dependent temperature T-hat,

Y. Zimmels

1997-01-01

379

Oxidation behavior of silicon carbide based biomorphic ceramics prepared by chemical vapor infiltration and reaction technique  

Microsoft Academic Search

The oxidation behavior of biomorphic SiC based ceramics with different microstructure and composition was studied at 1450°C in airflow for 50h by thermal gravimetric analysis (TGA). SiC with amorphous, coarse grain, crystalline and fine grain crystalline microstructures as well as SiC–Si3N4 composite ceramics were processed from paper preforms by chemical vapor infiltration and reaction technique. The ceramics were characterized by

H. Ghanem; E. Alkhateeb; H. Gerhard; N. Popovska

2009-01-01

380

Effectiveness of Conceptual Change Text-oriented Instruction on Students’ Understanding of Energy in Chemical Reactions  

Microsoft Academic Search

The aim of this study is to compare the effectiveness of conceptual change text instruction (CCT) in the context of energy\\u000a in chemical reactions. The subjects of the study were 60, 10th grade students at a high school, who were in two different\\u000a classes and taught by the same teacher. One of the classes was randomly selected as the experimental

Özgecan Tastan; Eylem Yalçinkaya; Yezdan Boz

2008-01-01

381

Polysulfone-polypyrrole ionic conductive composite membranes synthesized by phase inversion with chemical reaction  

Microsoft Academic Search

New polymeric composite membranes polysulfone-polypyrrole has been prepared by a new technique consisting in phase inversion accompanied by chemical reaction. The membranes were synthesised from polysulfone\\/pyrrole-N, N'-dimethylformamide\\/ methanol system by phase inversion followed by remaining pyrrole in membrane pores polymerization in the presence of iron chloride. The obtained membrane was characterized by Fourrier transformed infrared spectroscopy (FT-IR), Thermogravimetric analysis (TGA),

C. Muscalu; R. David; S. A. Garea; A. C. Nechifor; D. I. Vaireanu; S. I. Voicu; G. Nechifor

2009-01-01

382

Nonlinear Oscillations and Multiscale Dynamics in a Closed Chemical Reaction System  

Microsoft Academic Search

We investigate the oscillatory chemical dynamics in a closed isothermal reaction system described by the reversible Lotka–Volterra\\u000a model. This is a three-dimensional, dissipative, singular perturbation to the conservative Lotka–Volterra model, with the\\u000a free energy serving as a global Lyapunov function. We will show that there is a natural distinction between oscillatory and\\u000a non-oscillatory regions in the phase space, that is,

Yongfeng Li; Hong Qian; Yingfei Yi

2010-01-01

383

Low temperature chemical reactions in granitic and mafic aquifers: A laboratory study  

SciTech Connect

Column experiments were performed at 25[sup 0]C for 2,000 hours to simulate chemical reactions occurring in felsic and mafic aquifers. Batch experiments were performed in which each mineral was reacted at room temperature with 1 liter of distilled water for 5,000 hours to account for mineral contributions to the leachate chemistry. At the initial stage (< 1 hour) of the granite column experiment, leachate chemistry is dominated by Na[sup +] and HCO[sub 3][sup [minus

Moulton, K.L.; Veeger, A.I. (Univ. of Rhode Island, Kingston, RI (United States))

1992-01-01

384

Reactions of alkynes with [RuCl(cyclopentadienyl)] complexes: the important first steps.  

PubMed

Cyclopentadienyl-ruthenium half-sandwich complexes with eta(2)-bound alkyne ligands have been suggested as catalytic intermediates in the early stages of Ru-catalyzed reactions with alkynes. We show that electronically unsaturated complexes of the formula [RuCl(Cp--)(eta(2)-RC[triple bond]CR')] can be stabilized and crystallized by using the sterically demanding cyclopentadienyl ligand Cp-- (Cp--=eta(5)-1-methoxy-2,4-tert-butyl-3-neopentyl-cyclopentadienyl). Furthermore we demonstrate that [RuCl(2)(Cp==)](2) is an active and regioselective catalyst for the [2+2+2] cyclotrimerization of alkynes. The first elementary steps of the reaction of mono(eta(2)-alkyne) complexes containing {RuCl(Cp*)} (Cp*=eta(5)-C(5)Me(5)) and {RuCl(Cp--)} fragments with alkynes were investigated by DFT calculations at the M06/6-31G* level in combination with a continuum solvent model. Theoretical results are able to rationalize and complement the experimental findings. The presence of the sterically demanding Cp-- ligand increases the activation energy required for the formation of the corresponding di(eta(2)-alkyne) complexes, enhancing the initial regioselectivity, but avoiding the evolution of the system towards the expected cyclotrimerization product when bulky substituents are present. Theoretical results also show that the electronic structure and stability of a metallacyclic intermediate is strongly dependent on the nature of the substituents present in the alkyne. PMID:20583066

Dutta, Barnali; Curchod, Basile F E; Campomanes, Pablo; Solari, Euro; Scopelliti, Rosario; Rothlisberger, Ursula; Severin, Kay

2010-07-26

385

On the role of chemical reactions in initiating ultraviolet laser ablation in poly(methyl methacrylate)  

SciTech Connect

The role of chemical reactions is investigated versus the thermal and mechanical processes occurring in a polymer substrate during irradiation by a laser pulse and subsequent ablation. Molecular dynamics simulations with an embedded Monte Carlo based reaction scheme were used to study ultraviolet ablation of poly(methyl methacrylate) at 157 nm. We discuss the onset of ablation, the mechanisms leading to ablation, and the role of stress relaxation of the polymer matrix during ablation. Laser induced heating and chemical decomposition of the polymer substrate are considered as ablation pathways. It is shown that heating the substrate can set off ablation via mechanical failure of the material only for very short laser pulses. For longer pulses, the mechanism of ejection is thermally driven limited by the critical number of bonds broken in the substrate. Alternatively, if the photon energy goes towards direct bond breaking, it initiates chemical reactions, polymer unzipping, and formation of gaseous products, leading to a nearly complete decomposition of the top layers of substrates. The ejection of small molecules has a hollowing out effect on the weakly connected substrates which can lead to lift-off of larger chunks. Excessive pressure buildup upon the creation of gaseous molecules does not lead to enhanced yield. The larger clusters are thermally ejected, and an entrainment of larger polymer fragments in gaseous molecules is not observed.

Prasad, Manish; Conforti, Patrick F.; Garrison, Barbara J. [Department of Chemistry, 104 Chemistry Building, The Pennsylvania State University, University Park, Pennsylvania 16802 (United States)

2007-05-15

386

Tunable filtering of chemical signals in a simple nanoscale reaction-diffusion network.  

PubMed

We study numerically the filtering capabilities of a nanoscale network of two micrometer-sized containers joined by a nanotube, one of which hosts an enzymatic chemical reaction. Spatiotemporal chemical signals of substrate molecules are injected into the network. The substrate propagates by diffusion and reacts with enzymes distributed in the network prior to the injections. The dimensions of the network are tailored in a way that the transport and enzymatic reaction rates are comparable in size, a situation in which the overall behavior is highly influenced by the geometry and topology of the network. This property is crucial for the functionality of the filter developed in here. It is demonstrated that input signals can be classified in a crude way using a simple setup (a two-container network) and that the classification can be tuned by changing the geometry of the network (the length of the tube connecting the two containers). The filter device we investigate can also be viewed as a primitive chemistry-based computational element in the sense that the information encoded in the signals is processed using chemical reactions. In particular, it is demonstrated that the two-container device may filter out signals based on the average injection frequency. PMID:17497911

Lizana, L; Konkoli, Z; Orwar, O

2007-06-01

387

A large nonequilibrium effect of decrease of the bimolecular chemical reaction rate in a dilute gas  

NASA Astrophysics Data System (ADS)

The perturbation solution of the Boltzmann equation is used to analyze the relative decrease ?( T) of the rate constant k( T) of a chemical reaction in a dilute gas in the temperature T. The Prigogine-Xhrouet model of the reactive cross-section is introduced for the reaction A+A?B+B. A large effect for ?( T) equal even nearly 75% ( for a small reduced threshold energy ?0 and a large molar fraction xB) follows from the analytical solution for k( T). A replacement of T by the Shizgal-Karplus nonequilibrium temperatures TA and TB in the equilibrium rate constants (for the forward and reverse reactions) gives ?( TA, TB) confirming this effect.

Cukrowski, A. S.; Fritzsche, S.; Cukrowski, M. J.

2003-09-01

388

Efficient exact and K-skip methods for stochastic simulation of coupled chemical reactions  

NASA Astrophysics Data System (ADS)

Gillespie's direct method (DM) [D. Gillespie, J. Chem. Phys. 81, 2340 (1977)] for exact stochastic simulation of chemical reaction systems has been widely adopted. It is easy to implement but requires large computation for relatively large systems. Recently, two more efficient methods, next reaction method (NRM) [M. A. Gibson and J. Bruck, J. Phys. Chem. A 105, 1876 (2000)] and optimized DM (ODM) [Y. Cao et al., J. Chem. Phys. 121, 4059 (2004)], have been developed to improve simulation speed. It has been demonstrated that the ODM is the state-of-the-art most efficient method for exact stochastic simulation of most practical reaction systems. In this paper, we first develop an exact stochastic simulation algorithm named ODMK that is more efficient than the ODM. We then develop an approximate method named K-skip method to further accelerate simulation. Using two chemical reaction systems, we demonstrate that our ODMK and K-skip method can save 20%-30% and 70%-80% simulation time, respectively, comparing to the ODM. We also show that our ODMK and K-skip method provide almost the same simulation accuracy as the ODM.

Cai, Xiaodong; Wen, Ji

2009-08-01

389

Extension of a Kinetic-Theory Approach for Computing Chemical-Reaction Rates to Reactions with Charged Particles  

NASA Technical Reports Server (NTRS)

Recently introduced molecular-level chemistry models that predict equilibrium and nonequilibrium reaction rates using only kinetic theory and fundamental molecular properties (i.e., no macroscopic reaction rate information) are extended to include reactions involving charged particles and electronic energy levels. The proposed extensions include ionization reactions, exothermic associative ionization reactions, endothermic and exothermic charge exchange reactions, and other exchange reactions involving ionized species. The extensions are shown to agree favorably with the measured Arrhenius rates for near-equilibrium conditions.

Liechty, Derek S.; Lewis, Mark J.

2010-01-01

390

Accelerating the Computation of Detailed Chemical Reaction Kinetics for Simulating Combustion of Complex Fuels  

SciTech Connect

Combustion of hydrocarbon fuels has been a very challenging scientific and engineering problem due to the complexity of turbulent flows and hydrocarbon reaction kinetics. There is an urgent need to develop an efficient modeling capability to accurately predict the combustion of complex fuels. Detailed chemical kinetic models for the surrogates of fuels such as gasoline, diesel and JP-8 consist of thousands of chemical species and Arrhenius reaction steps. Oxygenated fuels such as bio-fuels and heavier hydrocarbons, such as from newer fossil fuel sources, are expected to have a much more complex chemistry requiring increasingly larger chemical kinetic models. Such models are beyond current computational capability, except for homogeneous or partially stirred reactor type calculations. The advent of highly parallel multi-core processors and graphical processing units (GPUs) promises a steep increase in computational performance in the coming years. This paper will present a software framework that translates the detailed chemical kinetic models to high- performance code targeted for GPU accelerators.

Grout, Ray W [ORNL

2012-01-01

391

Accelerating the Computation of Detailed Chemical Reaction Kinetics for Simulating Combustion of Complex Fuels  

SciTech Connect

Combustion of hydrocarbon fuels has been a very challenging scientific and engineering problem due to the complexity of turbulent flows and hydrocarbon reaction kinetics. There is an urgent need to develop an efficient modeling capability to accurately predict the combustion of complex fuels. Detailed chemical kinetic models for the surrogates of fuels such as gasoline, diesel and JP-8 consist of thousands of chemical species and Arrhenius reaction steps. Oxygenated fuels such as bio-fuels and heavier hydrocarbons, such as from newer fossil fuel sources, are expected to have a much more complex chemistry requiring increasingly larger chemical kinetic models. Such models are beyond current computational capability, except for homogeneous or partially stirred reactor type calculations. The advent of highly parallel multi-core processors and graphical processing units (GPUs) promises a steep increase in computational performance in the coming years. This paper will present a software framework that translates the detailed chemical kinetic models to high-performance code targeted for GPU accelerators.

Sankaran, R.; Grout, R.

2012-01-01

392

The chemical reactions in the human stomach and the relationship to metabolic disorders.  

PubMed

Acid hydrolysis of components from the diet in the stomach require the presence of an acid and a hydrolysing agent. The acid involved is hydrochloric acid. The present mechanism of hydrochloric acid production in the stomach is demonstrated to be incompatible with measured intracellular or intercellular concentrations of the relevant ions. An alternative set of chemical reactions whereby hydrochloric acid is formed in the stomach is presented. The hydrolysing agent is identified and a mechanism of transfer of chemical compounds into the metabolism is described. The hypothesis demonstrates that some of chemical compounds produced in the stomach can induce conditions such as asthma and that the conditions of osteoporosis and hemochromatosis can be linked to the function of the stomach. Possible treatments for these and other conditions such as stomach acidity and anaemia are proposed. PMID:15823700

Robertson, D S

2005-01-01

393

Simulation of gas absorption into string-of-beads liquid flow with chemical reaction  

NASA Astrophysics Data System (ADS)

This study is an attempt to investigate the chemical absorption of CO2 in aqueous monoethanolamine (MEA) solution in a wetted-wire column consisting of one wire. Computational fluid dynamics method along with volume of fluid model was employed for modeling of two-phase flow, mass transfer and chemical reaction inside the column. The modeling results were compared with available experimental data and very good agreement was achieved. The simulation results showed that the diameter and intervals of liquid beads increases by increasing the gas and liquid flow rates. The beads velocity increases by increasing the liquid flow rate and decreasing mass fraction of MEA in the liquid phase. Also, mass transfer resistance in the liquid phase reduces by formation of the beads. It was concluded that the developed model is capable to predict the effect of operating and physical parameters on the investigated chemical absorption process.

Hosseini, S. M.; Alizadeh, R.; Fatehifar, E.; Alizadehdakhel, A.

2014-04-01

394

Development of a model for predicting hydroxyl radical reaction rate constants of organic chemicals at different temperatures.  

PubMed

The reaction rate constants of hydroxyl radicals with organic chemicals (kOH) are of great importance for assessing the persistence and fate of organic pollutants in the atmosphere. However, experimental determination of kOH seems fairly unrealistic, due to the soaring number of the emerging chemicals additional to the large number of existing chemicals. Quantitative structure-activity relationship (QSAR) models are excellent choices for evaluating and predicting kOH values. In this study, a QSAR model that can predict kOH at different temperatures was developed by employing quantum chemical descriptors and DRAGON descriptors. The adjusted determination coefficient Radj(2) of the model is 0.873, and the external validation coefficient Qext(2) is 0.835, implying that the model has satisfactory robustness and good predictability. Additionally, a QSAR model was also built for kOH prediction at room-temperature (298 K). The development of the two models followed the guidelines for development and validation of QSAR models proposed by the Organization for Economic Co-operation and Development (OECD). The applicability domains of the current models were extended to several classes of compounds including long-chain alkenes (C8-C13), organophosphates, dimethylnaphthalenes, organic selenium and organic mercury compounds that have not been covered in the previous studies. PMID:24210594

Li, Chao; Yang, Xianhai; Li, Xuehua; Chen, Jingwen; Qiao, Xianliang

2014-01-01

395

Thermal decomposition reactions of HMX and RDX and their importance in predicting cookoff hazards  

SciTech Connect

To develop robust models for predicting the response of munitions under abnormal conditions associated with cookoff, it is necessary to be able to accurately characterize the following: the time to ignition, the location of the ignition point within the munition, and the combustive behavior of the damaged energetic material after ignition. For, the response of the munition, as controlled by these parameters, will determine whether its response will be characterized by a relatively mild deflagration or whether it will be characterized by a more damaging detonation. Several of the underlying properties of the energetic materials used in munitions that must be understood in order to accurately characterize these parameters are the chemical and physical changes that occur in these energetic materials as they are heated. The chemical changes involve overcoming the forces that tend to stabilize these materials, such as binding within the crystal lattice or intermolecular hydrogen bonding, and their transformation to less stable forms, such as mixtures of gases with high energy content. The physical changes typically involve phase changes of the material. One significant phase change is the slow transformation of the energetic materials from the solid reactant to gas phase products. This transformation can lead initially to the formation of high pressure gas bubbles within the solid particles and ultimately to changes in the porosity and gas permeability of the energetic material formulation. The presence of these reactive gases within high pressure bubbles can lead to increased hot spot formation of the material if it is compressed. The increased porosity can lead to significant increases in the burn rates of these materials at high pressures.

Behrens, R. Jr. [Sandia National Labs., Livermore, CA (United States); Bulusu, S. [Army Research and Development Command, Dover, NJ (United States)

1994-05-01

396

Novel duplex vapor: Electrochemical method for silicon solar cells. [chemical reactor for a silicon sodium reaction system  

NASA Technical Reports Server (NTRS)

The scaled up chemical reactor for a SiF4-Na reaction system is examined for increased reaction rate and production rate. The reaction system which now produces 5 kg batches of mixed Si and NaF is evaluated. The reactor design is described along with an analysis of the increased capacity of the Na chip feeder. The reactor procedure is discussed and Si coalescence in the reaction products is diagnosed.

Nanis, L.; Sanjurjo, A.; Sancier, K.

1979-01-01

397

On the importance of correcting for the uncompensated Ohmic resistance in model experiments of the oxygen reduction reaction.  

SciTech Connect

When measuring the current due to the Oxygen Reduction Reaction (ORR) and hydrogen oxidation reaction (HOR) on Pt and Pt alloys in aqueous electrolyte, it is important to take care of two major sources of error that are relatively easy to correct for. First, when measuring ORR voltammetry, adsorption processes are superimposed on the current. Second, the system resistance causes an Ohmic drop that may have a profound effect on the measured curves, especially at the higher currents close to the diffusion limiting current. More importantly, we show that it also influences the kinetic part of the potential curve in such a way that the Tafel slope may be determined incorrectly when failing to correct for Ohmic drop. Finally, because electrolyte resistance lowers with increasing temperature, failure to compensate for Ohmic.

van der Vliet, D.; Strmcnik, D. S.; Wang, C.; Stamenkovic, V. R.; Markovic, N. M; Koper, M. T. M.; Materials Science Division; Leiden Univ.

2010-08-15

398

Reactive removal of unstable mixed NO+CO adlayers: Chemical diffusion and reaction front propagation  

SciTech Connect

A lattice-gas model is developed to describe the reactive removal of a preadsorbed, mixed NO+CO adlayer covering a Pt(100) surface, via reduction of NO with CO, and behavior of the model is analyzed. Since NO dissociation requires an adjacent empty site, the NO+CO covered surface constitutes an unstable steady state. The creation of vacancies leads NO dissociation, the reaction of CO with the O formed by dissociation, the subsequent creation of more vacancies, and thus the autocatalytic removal of the adlayer. The high mobility of most adspecies leads to an initial {open_quotes}disperse stage{close_quotes} of adlayer removal, characterized by an exponential increase in the number of highly dispersed vacancies. Thereafter follows a transition to a {open_quotes}reaction front propagation{close_quotes} stage of adlayer removal, where a chemical wave develops that propagates into the NO+CO covered region of the surface with roughly constant velocity, and leaves in its wake a surface populated only by excess reactant. We provide a suitable rate equation formulation for the initial disperse stage, but focus on a reaction-diffusion equation analysis of reaction front propagation, examining, in detail, behavior for long times where the front is nearly planar. We emphasize that it is necessary to incorporate the coverage-dependent and tensorial nature of chemical diffusion in the mixed adlayer. Both these features reflect the interference on the surface diffusion of each adspecies by coadsorbed species. Thus, a key component of this work is the development of an appropriate treatment of chemical diffusion in mixed layers of several adspecies. {copyright} {ital 1998 American Institute of Physics.}

Tammaro, M. [Ames Laboratory and Department of Physics, Iowa State University, Ames, Iowa50011 (United States)] [Ames Laboratory and Department of Physics, Iowa State University, Ames, Iowa50011 (United States); Evans, J.W. [Ames Laboratory and Department of Mathematics, Iowa State University, Ames, Iowa50011 (United States)] [Ames Laboratory and Department of Mathematics, Iowa State University, Ames, Iowa50011 (United States)

1998-05-01

399

Importance of tetrahydrofolate and ATP in the anaerobic O-demethylation reaction for phenylmethylethers.  

PubMed

DL-Tetrahydrofolate (THF) and ATP were necessary for the anaerobic O-demethylation of phenylmethylethers in cell extracts of the type strain (ATCC 29683) of the homoacetogen Acetobacterium woodii. The reactants for this enzymatic activity have not been previously demonstrated in any system, nor has the mediating enzyme been studied. An assay using reaction mixtures containing 1 mM THF, 2 mM ATP, and 2 mM hydroferulate (i.e., 4-hydroxy,3-methoxyphenylpropionate) was developed and was performed under stringent anaerobic conditions. Pyridine nucleotides and several other possible cofactors were tested but had no effect on the activity. After centrifugation of disrupted cells at 27,000 x g, the activity was found primarily in the supernatant, which had a specific activity of 14.2 +/- 0.5 nmol/min/mg of protein. At saturating levels of each of the other two substrates, apparent Km values for the variable substrate were 0.65 mM hydroferulate, 0.27 mM ATP, and 0.17 mM THF. Activity was significantly decreased when extract was preincubated at 60 degrees C and was completely lost after preincubation in air for 30 min. Thus, the soluble anaerobic O-demethylating enzyme system of A. woodii is oxygen sensitive. The THF- and ATP-dependent activity measurable in the soluble fraction of cell extracts constituted about 34% of the activity seen with intact cells. PMID:1575495

Berman, M H; Frazer, A C

1992-03-01

400

Some reactions and properties of nitro radical-anions important in biology and medicine.  

PubMed

Nitroaromatic compounds, ArNO2 have widespread actual or potential use in medicine and cancer therapy. There is direct proof that free-radical metabolites are involved in many applications, and an appreciation of the conceptual basis for their therapeutic differential; however, an understanding of the detailed mechanisms involved is lacking. Redox properties control most biological responses of nitro compounds, and the characteristics of the one-electron couple: ArNO2/ArNO2- are detailed. The "futile metabolism" of nitroaryl compounds characteristic of most aerobic nitroreductase systems reflects competition between natural radical-decay pathways and a one-electron transfer reaction to yield superoxide ion, O2-. Prototropic properties control the rate of radical decay, and redox properties control the rate of electron transfer to O2 or other acceptors. There are clear parallels in the chemistry of ArNO2- and O2-. While nitro radicals have frequently been invoked as damaging species, they are very unreactive (except as simple reductants). It seems likely that reductive metabolism of nitroaryl compounds, although generally involving nitro radical-anions as obligate intermediates (and this is required for therapeutic selectivity towards anaerobes), results in biological damage via reductive metabolites of higher reduction order than the one-electron product. PMID:3830700

Wardman, P

1985-12-01

401

Some reactions and properties of nitro radical-anions important in biology and medicine.  

PubMed Central

Nitroaromatic compounds, ArNO2 have widespread actual or potential use in medicine and cancer therapy. There is direct proof that free-radical metabolites are involved in many applications, and an appreciation of the conceptual basis for their therapeutic differential; however, an understanding of the detailed mechanisms involved is lacking. Redox properties control most biological responses of nitro compounds, and the characteristics of the one-electron couple: ArNO2/ArNO2- are detailed. The "futile metabolism" of nitroaryl compounds characteristic of most aerobic nitroreductase systems reflects competition between natural radical-decay pathways and a one-electron transfer reaction to yield superoxide ion, O2-. Prototropic properties control the rate of radical decay, and redox properties control the rate of electron transfer to O2 or other acceptors. There are clear parallels in the chemistry of ArNO2- and O2-. While nitro radicals have frequently been invoked as damaging species, they are very unreactive (except as simple reductants). It seems likely that reductive metabolism of nitroaryl compounds, although generally involving nitro radical-anions as obligate intermediates (and this is required for therapeutic selectivity towards anaerobes), results in biological damage via reductive metabolites of higher reduction order than the one-electron product.

Wardman, P

1985-01-01

402

The Importance of IgG Avidity and the Polymerase Chain Reaction in Treating Toxoplasmosis during Pregnancy: Current Knowledge  

PubMed Central

A brief report on the nature and epidemiology of T. gondii infection is firstly presented. The importance of the specific IgG avidity test and polymerase chain reaction (PCR) for toxoplasmosis is discussed, along with their significance and importance as auxiliary methods for determining the most likely time for the initial infection by this coccidian and for defining the therapeutic strategy. Lastly, practical comments are made in relation to the classical therapeutic regimens, with special attention to the indications for fetal treatment, when this is necessary.

Bortoletti Filho, Joao; Carvalho, Natalia da Silva; Helfer, Talita Micheletti; Nogueira Serni, Priscila de Oliveira; Nardozza, Luciano Marcondes Machado; Moron, Antonio Fernandes

2013-01-01

403

Probabilistic Health Risk Assessment of Chemical Mixtures: Importance of Travel Times and Connectivity  

NASA Astrophysics Data System (ADS)

Subsurface contamination cases giving rise to groundwater pollutions are extensively found in all industrialized countries. Under this pressure, risk assessment methods play an important role in population protection by (1) quantifying the potential impact on human health of an aquifer contamination and (2) helping and driving decisions of groundwater-resource managers. Many reactive components such as chlorinated solvents or nitrates potentially experience attenuation processes under common geochemical conditions. This represents an attractive and extensively used remediation solution but leads often to the production of by-products before to reach a harmless chemical form. This renders mixtures of contaminants a common issue for groundwater resources managers. In this case, the threat posed by these contaminants to human health at a given sensitive location greatly depends on the competition between reactive and advective-dispersive characteristic times. However, hydraulic properties of the aquifer are known to be spatially variable, which can lead to the formation of preferential flow channels and fast contamination pathways. Therefore, the uncertainty on the spatial distribution of the aquifer properties controlling the plume travel time may then play a particular role in the human health risk assessment of chemical mixtures. We investigate here the risk related to a multispecies system in response to different degrees of heterogeneity of the hydraulic conductivity (K or Y =ln(K)). This work focuses on a Perchloroethylene (PCE) contamination problem followed by the sequential first-order production/biodegradation of its daughter species Trichloroethylene (TCE), Dichloroethylene (DCE) and Vinyl Chlorine (VC). For this specific case, VC is known to be a highly toxic contaminant. By performing numerical experiments, we evaluate transport through three-dimensional mildly (?Y 2=1.0) and highly (?Y 2=4.0) heterogeneous aquifers. Uncertainty on the hydraulic conductivity field is considered through a Monte Carlo scheme, and statistics of the total risk for human health (RT) related to the mixtures of the four carcinogenic plumes are evaluated. Results show two distinct spatiotemporal behavior of the RT estimation. Simulations in highly heterogeneous aquifers display a lower mean of RT close to the injection and higher further away. We explain this by the distinct ranges of travel times and connectivity metrics related to the two sets of aquifers. A high ?Y 2 trends to decrease the travel time (and increase the connectivity). Early travel times, associated to channeling effects, are intuitively perceived as an indicator for high risk. However, in our case, early travel times lead a limited production of highly toxic daughter species and a lower total risk. Our results reflect then the interplay between the characteristic reactive time for each component and the characteristic travel time of the plume since the production of VC depends on these factors.

Henri, Christopher V.; Fernàndez-Garcia, Daniel; de Barros, Felipe P. J.

2014-05-01

404

Methyl salicylate: a reactive chemical warfare agent surrogate to detect reaction with hypochlorite.  

PubMed

Methyl salicylate (MeS) has a rich history as an inert physical simulant for the chemical warfare agents sulfur mustard and soman, where it is used extensively for liquid- and vapor-permeation testing. Here we demonstrate possible utility of MeS as a reactivity simulant for chlorine-based decontaminants. In these experiments MeS was reacted with sodium hypochlorite varying stoichiometry, temperature, reaction time, and pH. No colored oxidation products were observed; however, chlorination of the aromatic ring occurred ortho (methyl 3-chlorosalicylate) and para (methyl 5-chlorosalicylate) to the position bearing the -OH group in both the mono- and disubstituted forms. The monosubstituted para product accumulated initially, and the ortho and 3,5-dichloro products formed over the next several hours. Yields from reactions conducted below pH 11 declined rapidly with decreasing pH. Reactions run at 40 °C produced predominantly para substitution, while those run at 0 °C produced lower yields of ortho- and para-substituted products. Reactions were also carried out on textile substrates of cotton, 50/50 nylon-cotton, and a meta aramid. The textile data broadly reproduced reaction times and stoichiometry observed in the liquid phase, but are complicated by physical and possibly chemical interactions with the fabric. These data indicate that, for hypochlorite-containing neutralizing agents operating at strongly alkaline pH, one can expect MeS to react stoichiometrically with the hypochlorite it encounters. This suggests utility of MeS in lieu of such highly hazardous surrogates as monochloroalkyl sulfides as a simulant for threat scenarios involving the stoichiometric decomposition of sulfur mustard. Specifically, the extent of coverage of the simulant on a fabric by the neutralizing agent can be directly measured. Similar reactivity toward other halogen oxidizing agents is likely but remains to be demonstrated. PMID:21981047

Salter, W Bruce; Owens, Jeffery R; Wander, Joseph D

2011-11-01

405

Hydrodynamic limits for the monomer-dimer surface reaction: Chemical diffusion, wave propagation, and equistability  

SciTech Connect

For finite adspecies mobility, the lattice-gas monomer-dimer (A+B{sub 2}) surface reaction model exhibits a discontinuous transition from a stable reactive steady state to a stable A-poisoned steady state, as the impingement rate P{sub A} for A increases above a critical value P{sup {asterisk}}. The reactive (poisoned) state is metastable for P{sub A} just above (below) P{sup {asterisk}}. Increasing the surface mobility of A enhances metastability, leading to bistability in the limit of high mobility. In the bistable region, the more stable state displaces the less stable one separated from it by a planar interface, with P{sup {asterisk}} becoming the equistability point for the two states. This hydrodynamic regime can be described by reaction-diffusion equations (RDE{close_quote}s). However, for finite reaction rates, mixed adlayers of A and B are formed, resulting in a coverage-dependent and tensorial nature to chemical diffusion (even in the absence of interactions beyond site blocking). For equal mobility of adsorbed A and B, and finite reaction rate, the prediction for P{sup {asterisk}} from such RDE{close_quote}s, incorporating the appropriate description of chemical diffusion, is shown to coincide with that from kinetic Monte Carlo simulations for the lattice-gas model in the regime of high mobility. Behavior for this special case is compared with that for various other prescriptions of mobility, for both finite and infinite reaction rates. {copyright} {ital 1998} {ital The American Physical Society}

Tammaro, M. [Department of Physics, University of Rhode Island, Kingston, Rhode Island02881 (United States)] [Department of Physics, University of Rhode Island, Kingston, Rhode Island02881 (United States); Evans, J.W. [Ames Laboratory and Department of Mathematics, Iowa State University, Ames, Iowa50011 (United States)] [Ames Laboratory and Department of Mathematics, Iowa State University, Ames, Iowa50011 (United States)

1998-05-01

406

Some new reaction pathways for the formation of cytosine in interstellar space - A quantum chemical study  

NASA Astrophysics Data System (ADS)

The detection of nucleic acid bases in carbonaceous meteorites suggests that their formation and survival is possible outside of the Earth. Small N-heterocycles, including pyrimidine, purines and nucleobases, have been extensively sought in the interstellar medium. It has been suggested theoretically that reactions between some interstellar molecules may lead to the formation of cytosine, uracil and thymine though these processes involve significantly high potential barriers. We attempted therefore to use quantum chemical techniques to explore if cytosine can possibly form in the interstellar space by radical-radical and radical-molecule interaction schemes, both in the gas phase and in the grains, through barrier-less or low barrier pathways. Results of DFT calculations for the formation of cytosine starting from some of the simple molecules and radicals detected in the interstellar space are being reported. Global and local descriptors such as molecular hardness, softness and electrophilicity, and condensed Fukui functions and local philicity indices were used to understand the mechanistic aspects of chemical reaction. The presence and nature of weak bonds in the molecules and transition states formed during the reaction process have been ascertained using Bader's quantum theory of atoms in molecules (QTAIMs). Two exothermic reaction pathways starting from propynylidyne (CCCH) and cyanoacetylene (HCCCN), respectively, have been identified. While the first reaction path is found to be totally exothermic, it involves a barrier of 12.5 kcal/mol in the gas phase against the lowest value of about 32 kcal/mol reported in the literature. The second path is both exothermic and barrier-less. The later has, therefore, a greater probability of occurrence in the cold interstellar clouds (10-50 K).

Gupta, V. P.; Tandon, Poonam; Mishra, Priti

2013-03-01

407

Relative importance of dynamical and chemical contributions to Arctic wintertime ozone  

NASA Astrophysics Data System (ADS)

We present the first complete budget of the interannual variability in Arctic springtime ozone taking into account anthropogenic chemical and natural dynamical processes. For the winters 1991/1992 to 2003/2004 the Arctic chemical ozone loss is available from observations. This work investigates the dynamical supply of ozone to the Arctic polar vortex due to mean transport processes for the same winters. The ozone supply is quantified in a vortex-averaged framework using estimates of diabatic descent over winter. We find that the interannual variability of both dynamical ozone supply and chemical ozone loss contribute, in equal shares, to the variability of the total ozone change. Moreover, together they explain nearly all of the interannual variability of Arctic springtime column ozone. Variability in planetary wave activity, characterized by the Eliassen-Palm flux at 100 hPa, contributes significantly to the variability of ozone supply, chemical ozone loss and total springtime ozone.

Tegtmeier, S.; Rex, M.; Wohltmann, I.; Krüger, K.

2008-09-01

408

Theoretical study of the dynamics of multi-dimensional systems: Vibrationally excited chemical reaction and the denaturation of DNA  

NASA Astrophysics Data System (ADS)

Many chemical dynamical processes involve the behavior of a large number of coupled degrees of freedom. It has been a challenge to describe them accurately using theory. By developing practical methodologies to simplify the systems and the processes, we have studied two kinds of interesting systems: vibrationally excited chemical reaction dynamics and the denaturation of double helix DNA. First we studied the chemical reaction dynamics between an atom and a tri-atomic molecule, using a quasiclassical trajectory method. The equations of motion are solved by classical mechanics, with the potential energy surface fit from ab initio calculations, and the initial and final vibrational-rotational states of the diatomic and tri-atomic molecules quantized by a semi-classical method. We have studied the dynamics of two benchmark systems, the reactions H + H2O ? H2 + OH and H + HCN ? H2 + CN in detail, with focus on the enhancement of reactivity from vibrational excitation of the tri-atomic reactant. Most of our results are in very good agreement with state-of-art experimental data. For the reaction between H and H2O, we discovered that with different OH stretching excitation of H2O there is a transition in the reaction dynamics from activated to activationless behavior. The thermal denaturation (melting) of double helix DNA is one of the most important physiological processes. When DNA molecules are attached to gold nanoparticles, the melting temperature becomes higher and the melting curve becomes much sharper than with DNA in solution. In order to study this complicated process, we have developed a reduced model, with each nucleotide simplified as one backbone site and one hydrogen bonding site, and used empirical potential functions to simulate the denaturation by Langevin dynamics. Our results on the melting of some short length DNA molecules, including the melting temperature and width agree well with experimental data. In addition, we found that for the melting of nanoparticle linked DNA, multiple melting steps may be involved. A cooperative thermodynamical model can be developed which fits the experimental results accurately.

Wu, Guosheng

409

Precision synthesis of functional materials via RAFT polymerization and click-type chemical reactions  

NASA Astrophysics Data System (ADS)

The need to tailor polymeric architectures with specific physico-chemical properties via the simplest, cleanest, and most efficient synthetic route possible has become the ultimate goal in polymer synthesis. Recent progress in macromolecular science, such as the discoveries of controlled/"living" free radical polymerization (CRP) methods, has brought about synthetic capabilities to prepare (co)polymers with advanced topologies, predetermined molecular weights, narrow molecular weight distributions, and precisely located functional groups. In addition, the establishment of click chemistry has redefined the selected few highly efficient chemical reactions that become highly useful in post-polymerization modification strategies. Hence, the ability to make well-defined topologies afforded by controlled polymerization techniques and the facile incorporation of functionalities along the chain via click-type reactions have yielded complex architectures, allowing the investigation of physical phenomena which otherwise could not be studied with systems prepared via conventional methods. The overarching theme of the research work described in this dissertation is the fusion of the excellent attributes of reversible addition-fragmentation chain transfer (RAFT) polymerization method, which is one of the CRP techniques, and click-type chemical reactions in the precision of synthesis of advanced functional materials. Chapter IV is divided into three sections. In Section I, the direct RAFT homopolymerization of 2-(acryloyloxy)ethyl isocyanate (AOI) and subsequent post-polymerization modifications are described. The polymerization conditions were optimized in terms of the choice of RAFT chain transfer agent (CTA), polymerization temperature and the reaction medium. Direct RAFT polymerization of AOI requires a neutral CTA, and relatively low reaction temperature to yield AOI homopolymers with low polydispersities. Efficient side-chain functionalization of PAOI homopolymers was achieved via reaction with model amine, thiol and alcohol compounds yielding urea, thiourethane and urethane derivatives, respectively. Reactions with amines and thiols (in the presence of base) were rapid, quantitative and efficient. However, the reaction with alcohols catalyzed by dibutyltin dilaurate (DBTDL) was relatively slow but proceeded to completion. Selective reaction pathways for the addition of difunctional ethanolamine and mercaptoethanol were also investigated. A related strategy is described in Section II wherein a hydroxyl-containing diblock copolymer precursor was transformed into a library of functional copolymers via two sequential post-polymerization modification reactions. A diblock copolymer scaffold, poly[(N,N-dimethylacrylamide)-b-( N-(2-hydroxyethyl)acrylamide] (PDMA-b-PHEA) was first prepared. The hydroxyl groups of the HEA block were then reacted with 2-(acryloyloxy)ethylisocyanate (AOI) and allylisocyanate (AI) resulting in acrylate- and allyl-functionalized copolymer precursors, respectively. The efficiencies of Michael-type and free radical thiol addition reactions were investigated using selected thiols having alkyl, aryl, hydroxyl, carboxylic acid, amine and amino acid functionalities. The steps of RAFT polymerization, isocyanate-hydroxyl coupling and thiol-ene addition are accomplished under mild conditions, thus offering facile and modular routes to synthesize functional copolymers. The synthesis and solution studies of pH- and salt-responsive triblock copolymer are described in Section III. This system is capable of forming self-locked micellar structures which may be controlled by changing solution pH as well as ionic strength. A triblock copolymer containing a permanently hydrophilic poly(N,N-dimethylacrylamide) (PDMA) outer block, a salt-sensitive zwitterionic poly(3[2-(N-methylacrylamido)ethyl dimethylammonio]propanesulfonate) (PMAEDAPS) middle block and a pH-responsive 3-acrylamido-3-methylbutanoic acid (PAMBA) core block was synthesized using aqueous RAFT polymerization. A facile formation of "self-locking&quo

Flores, Joel Diez

410

Heat Integration of the Water-Gas Shift Reaction System for Carbon Sequestration Ready IGCC Process with Chemical Looping  

SciTech Connect

Integrated gasification combined cycle (IGCC) technology has been considered as an important alternative for efficient power systems that can reduce fuel consumption and CO2 emissions. One of the technological schemes combines water-gas shift reaction and chemical-looping combustion as post gasification techniques in order to produce sequestration-ready CO2 and potentially reduce the size of the gas turbine. However, these schemes have not been energetically integrated and process synthesis techniques can be applied to obtain an optimal flowsheet. This work studies the heat exchange network synthesis (HENS) for the water-gas shift reaction train employing a set of alternative designs provided by Aspen energy analyzer (AEA) and combined in a process superstructure that was simulated in Aspen Plus (AP). This approach allows a rigorous evaluation of the alternative designs and their combinations avoiding all the AEA simplifications (linearized models of heat exchangers). A CAPE-OPEN compliant capability which makes use of a MINLP algorithm for sequential modular simulators was employed to obtain a heat exchange network that provided a cost of energy that was 27% lower than the base case. Highly influential parameters for the pos gasification technologies (i.e. CO/steam ratio, gasifier temperature and pressure) were calculated to obtain the minimum cost of energy while chemical looping parameters (oxidation and reduction temperature) were ensured to be satisfied.

Juan M. Salazara; Stephen E. Zitney; Urmila M. Diwekara

2010-01-01

411

On-chip isothermal, chemical cycling polymerase chain reaction (ccPCR)  

NASA Astrophysics Data System (ADS)

We demonstrate a novel ccPCR technique for microfluidic DNA amplification where temperature is held constant in space and time. The polymerase chain reaction is a platform of choice for biological assays and typically based on a three-step thermal cycling: DNA denaturation, primers annealing and extension by an enzyme. We here demonstrate a novel technique where high concentration chemical denaturants (solvents) denature DNA. We leverage the high electrophoretic mobility of DNA and the electrical neutrality of denaturants to achieve chemical cycling. We focus DNA with isotachophoresis (ITP); a robust electrophoretic preconcentration technique which generates strong electric field gradients and protects the sample from dispersion. We apply a pressure-driven flow to balance electromigration velocity and keep the DNA sample stationary in a microchannel. We drive the DNA through a series of high denaturant concentration zones. DNA denatures at high denaturant concentration. At low denaturant concentration, the enzyme creates complementary strands. DNA reaction kinetics are slower than buffer reactions involved in ITP. We demonstrate successful ccPCR amplification for detection of E. Coli. The ccPCR has the potential for simpler chemistry than traditional PCR.

Persat, Alexandre; Santiago, Juan

2008-11-01

412

A Molecular Dynamics Study of Chemical Reactions of Solid Pentaerythritol Tetranitrate at Extreme Conditions  

SciTech Connect

We have carried out density functional based tight binding (DFTB) molecular dynamics (MD) simulation to study energetic reactions of solid Pentaerythritol Tetranitrate (PETN) at conditions approximating the Chapman-Jouguet (CJ) detonation state. We found that the initial decomposition of PETN molecular solid is characterized by uni-molecular dissociation of the NO{sub 2}groups. Interestingly, energy release from this powerful high explosive was found to proceed in several stages. The large portion of early stage energy release was found to be associated with the formation of H{sub 2}O molecules within a few picoseconds of reaction. It took nearly four times as long for majority of CO{sub 2} products to form, accompanied